Workaround for Exchange 2010 & Outlook 2003 Shared Calendars Connectivity Issues “The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.”

This is a rather long blog about an issue we had during an Exchange 2007 to Exchange 2010 transition project @ a 1.500 mail boxes sized company. This was last summer but I needed to find the time to blog this so that it might help people who also need to find a solution. We ran in to this very annoying situation for users who still have outlook 2003 (SP3, fully patched) and were moved over to Exchange 2010 Roll Up 4. The users have rather a lot of shared calendars open & after accessing some of them (the sweet spot seemed to be around 6 to 7) they got this error:

"The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action."

Edit November 15th 2010: Microsoft released a KB article titled  “Error message when Outlook 2003 clients try to open multiple shared calendars in Exchange Server 2010: "The connection to the Microsoft Exchange server in unavailable. Outlook must be online or connected to complete this action" on this issue that is based on the work done on this case. The support engineer who worked on this case with me notified me by mail that the article had been released. Thanks Kovai! They only mention the throttling Policy as the /resetnavpane is not always required, this depends on the environment history. 

Edit February 3th 2011: The support engineer published a very content rich blog about all this and other possible causes of the error notification on his TechNet blog: Things you need to know about “The connection to the Microsoft Exchange server in unavailable. Outlook must be online or connected to complete this action” prompts in an Outlook 2003–Exchange 2010 world. It’s a must read for all people dealing with this error message.

When the user closes and reopens Outlook 2003 access sometimes worked for the calendar that threw the error before but than other calendars throw the error. So it becomes game of closing and opening Outlook 2003 in the hope you can open the desired calendar. This does not make users very happy as you can imagine. We stopped the migration to Exchange 2010 for users still on Outlook 2003. On the Exchange 2010 forums you’ll find and which both discuss this problem. It’s also pretty random. Sometimes it’s a shared calendar of a user on Exchange 2010 or on Exchange 2007. There is also a mention of this issue on others sites and one of them mentioned a private discussion with Microsoft about a fix coming in E2K10 Roll Up 5.I asked MS support later on if they had any knowledge of this and they said that they had none what so ever. By the way SP1 for Exchange 2010 arrived before we even got to roll up 5 for Exchange 2010 RTM.

We investigated and searched for a solution. One very promising work around was related to the throttling mechanism in Exchange 2010.

See also for more information on this. This is a good right up as well:

The default throttling policy limits the RPC connections to 20. Now when Outlook 2003 opens a shared calendar it consumes a RPC connection. It doesn’t release that until outlook is closed. We taught we had a winner here as this could lead to the error while sending and receiving mail keeps working.

So in an attempt to fix the problem we tried this:

New-ThrottlingPolicy –name Outlook2003Calendar

Set-ThrottlingPolicy –identity Outlook2003Calendar –RCAMaxConcurrency 100

Set-Mailbox –Identity “annoyed user” –ThrottlingPolicy Outlook2003Calendar

We’re supposed to have some patience before this would work, due to Active Directory replication so we did. We even left it overnight. No joy. We also tried disabling throttling by using $NULL. Unfortunately this didn’t work either. We verified if the throttling was indeed the issue by counting the connections for a troublesome Outlook 2003 user and found that the error even occurred below the throttle limit. You can figure out the number of RPC connections by using:

Get-LogonStatistics –Identity <annoyed usser> | fl applicationid

Well we ran out of ideas (can you believe that?!) and so we called Microsoft Support to log a case. One of the goodies with our TechNet subscriptions via Software Assurance is that we have free support calls! Open-mouthed

The feedback was not coming fast. We also did not get any requests for more information. Not good sign. And when we pinged ‘m for info they acknowledged the bug and told us that it would probably be fixed in Exchange 2010 SP1. They were not 100% certain that our issue was due to this bug. But we did not get any request for further information or diagnostics. They asked us to go to Exchange 2007 SP3. At the time we were on Exchange 2007 SP2 Roll Up 4. There was not really an indication this would fix anything. The users who had the issue were the ones who had been moved to Exchange 2010. But we went along, put in the overtime past office hours and we went to Exchange 2007 SP3. As expected this did not help at all.

Then finally advanced support came in to play, now that was a different ballgame. Lots of questions request for logs, network traces, executing tests both client side and server side related. The engineer was really engaged and was working hard on this. He has my thanks and it took some time to do all the testing and work trough the results.

We found out some interesting things. When a user with a mailbox on Exchange 2007 or 2010 opens his shared calendars using Outlook 2003 the list is different than what you see in OWA or using Outlook 2007 and Outlook 2010. This is due to changes in the way those shared calendars are accessed. I was not able to find out any more details on the subject but this could be due to the fact that Outlook 2003 by default used referenced MDB model when additional calendars/mailboxes were opened. This feature isn’t supported in Exchange 2010 and due to the underlying design server side changes outlook 2003 now establishes more connections than it did with previous exchange versions. I assume it was still supported in Exchange 2007.

With Exchange 2010 the combination of the throttling policy and the changes in how shared calendars are accessed and the fact they contain “persistence information” caused this issue. We created a new throttling policy and i was necessary to delete all shared calendars from outlook, close outlook, start outlook and add them again. Manually this is done one calendar at the time and I pretty tedious. At least you can speed up things by using outlook.exe /resetnavpane. This will delete all the shared calendars for you and saves a lot of time. It also resets the entire navigation pane so any other “issues” lingering around are thrown out as well.

In summary (this an adaption of the support case conclusions. The escalation engineer working on the case was a great guy and he really put in an effort).


  • When Outlook 2003 clients whose mailbox is on Exchange 2010 try to open additional calendars (on Exchange 2010 or on Exchange 2007) they get error popup "The connection to the Microsoft Exchange server in unavailable. Outlook must be online or connected to complete this action"


  • The popups might happen while opening any additional calendar , no specific pattern on occurrence of pop up message based on specific number of additional calendars opened or a specific additional calendar/calendars being opened.
  • The issue didn’t happen when the mailboxes were on Exchange 2007, issue doesn’t happen with versions of outlook higher than Outlook 2003.
  • All the concerned mailboxes are on Exchange 2010.
  • There is an indication that mailboxes that existed in the organization since Exchange 2000 days face the problem. Mailboxes that were created since the introduction of Exchange 2007 and never had a mailbox prior to that version didn’t have the problem.


  1. While the mailbox is on Exchange 2010, outlook 2003 will use Exchange 2010’s Address Book service to query the legacyDN of target shared mailboxes/calendars that needs to be opened and Exchange 2010 returns this as mungedDNs (not typical legacy exchange DNs) which forces outlook 2003 to think it’s a new server every time and establish more connections so eventually the default allowed maximum of 20 connections would exhaust.
  2. Outlook 2003 maintains hidden persistence messages containing various information about the shared mailboxes/calendars, such as the user name and the server where the mailbox resides. Thus it doesn’t have updated information and could very well reach a non-existing server as well.


  1. For the cause1, we set up a custom throttling policy with RCAMaxConcurrency set at 100 and applied this policy on mailboxes still using Outlook 2003.  For the change to take effect immediately, we need to force AD replication, restart of throttling service, RPCClientAccess service on CAS and MBX servers
  2. For the cause2, we either need to remove those stale entries and re-add the shared calendar entries (need to sync the deletion with server, then close and reopen outlook prior to re-adding entries again) or run ‘outlook.exe /resetnavpane’ switch.

Most issues with Outlook 2003 are well documented in KB articles and on the Microsoft Exchange Team Blog ( but not this one and it hurt us when the users complaints came in. We did not catch this one in the labs prior to the transition. We can’t win ‘m all, I know that. But the most obvious solution, upgrade to Outlook 2007/2010, is often not an acceptable option for financial, timing, practical and compatibility reasons. A couple of weeks ago I saw a tweet by Jetze Mellema about what he read somewhere “Friends don’t let friends upgrade to Exchange 2010 when using Outlook 2003.” I wouldn’t go that far. But this shared calendar issue should have been documented by now and made public I think.

63 thoughts on “Workaround for Exchange 2010 & Outlook 2003 Shared Calendars Connectivity Issues “The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.”

  1. thanks for this. well written, thorough and extremely useful. we got as far as adjusting throttling, but were stumped on the second part.

  2. Good article and thanks for dharing. We’re migrating from Exch 2003 to Exch 2010 and most users are still on Outlook 2003. Exact same problem. Many thanks.

      • In addition I found that lots of users were experiencing this seemingly random unavailability of shared calendars, regardless of where their mailbox was homed or what version of the client they were using. I discovered this hot fix:

        The description attached to this article does not mention the shared calendars exactly and seems only to apply to other delegate access. After installing this on all of my Exchange 2003 servers (restart of Exchange SA was necessary) this resolved a lot of the random problems. Users homed on Exchange 2003 could access previously inaccessible calendars on mailbox on both Exchange 2003 and 2010.

        After this the original problem was isolated to Outlook 2003 users attempting to access the calendars that they had listed in the “Shared Calendars” list in their Outlook 2003 navigation pane. Using the and adding the calendars back solved this completely.

        • Hi, yes we saw this one to. But in our cases no Exchange 2003 servers where involved. So we didn’t look into it any further. Good you mention it for others who might be dealing with these issues and thank you.

      • Hello Expert,
        We have migrated exchange 2007 sp3 to exchange 2010 sp1.Later we have upgraded exchange 2010 sp1 to 2010 sp3.
        The client outlook 2010,2013 are having the issues while sharing the calender even after assigned user permission.
        When I was used exchange 2007 with outlook 2007 the calender sharing was pretty easy.

        Please help me I am wondering why the outlook 2010 and outlook 2013 is having issues with exchange 2010.


  3. Thanks for a great article. The suggested changes did not fix our issue. It wasn’t until I removed the old global catalog from our old Exchange 2007 server that it started working correctly.

  4. Hello.

    Thanks for your article.

    We have 2010 SP1 with Outlook 2003, and have been experiencing all the issues listed above.

    However, even after setting our new throttlingpolicy to $null for RCA and EWS settings, Outlook still refused to open multiple connections.

    We have discovered an add-in on Outlook was causing our problems, Kaspersky Mail Checker.

    Disabling this dll and uninstalling the package allowed users to open their additional mailboxes and calendars correctly after all.

    Thanks for your help though!

    • Hi Martin,

      You’re absolutely right! When you search for this error you’ll find recomendations to check for problematic add-ins. We also looked a the anti virus and other possible add-ins. I totally forgot to mention this, but I’m glad you pointed it out. Thanks.

  5. Well the saga continues with us I’m afraid…

    After weeks of testing, the issue is back again, but this time causing more random issues with Additional mailboxes, Calendars and Attaching items to Calendar entries in peoples own mailboxes.

    This is even affecting users who have one or two shared calendars, not just our heavy users.

    Throttling Policy has been set to $null for the above, and we have even applied our BESPolicy to the affected users to no avail (we can verify the BESAdmin account is fine with multiple mailboxes)

    We have noticed, if you Open Outlook, and quickly check all of the shared calendars you can get into them all, but if you were to stagger the clicking slowly, you get to a certain point and can open no more.

    Also, Outlook 2003 seems to pre-open all of the additional mailboxes when Outlook closes, and again, you can see the unread count go down the list until a certain point, where again the connection is lost and the above errors return until you close Outlook and re-open.

    Any further ideas?
    All Plugins have been disabled, and

  6. Hello

    Thanks again, we were seeing this Event ID and it seems like the people affected were the ones in the events. I will make the reg changes and post back results.

    Best Regards and Happy New Year

  7. I just wanted to say thank you for posting this. We recently moved from Exchange 2003 to Exchange 2010 SP1. I have been on the phone with Microsoft Support for the last 2.5 days to no resolve. Granted, they were very close in solving this. One of the RPC engineers had me set the MAXconcurreny value to 40, but this didn’t work (reason being, the user had anywhere form 42-74 rpc connections at that time). We also tried $null as a value, however we would see errors in the Application log stating they were “over budget” for the default policy, even though they were using the new policy we created for the user.

    None the less, I eventually stumbled on this blog:
    which lead me to this page and ultimately to the resolution of this problem (as far as I can tell at this time).

    Also to note, we encounted the Mapi sessions error and resolved that as well. The registry key name change through us off.

    Thanks again for taking the time to post this. I’m really amazed there is not more information about this or that Microsoft has not addressed this.

    Best Regards

  8. Thanks for the great article!
    I do have a question though – what about people using BES5, would changing the throttling affect the BES users since Exchange 2010 SP1 requires a $null setting?


    • It seems to me like BES users are allready good to go? It’s $Null as required by BES/E2K10SP1 so for the other uses like shared calendars I think you can just leave it that way for thos BES users, there is no limit. In RTM a lot of Exchange admins with BES allready used -RCAMaxConcurrency $null or upped it to 100. Exchange 2010 SP1, MaxSessionsPerUser does not exist anymore. This was used for the addressbook service and is now also governed by RCA* properties. So when you’ve done what’s needed for BES in E2K10SP1 you should be good for the problem with shared calendaring discussed here.

  9. Fantastic Article. Microsoft should publish something about this. I had a ticket open with MS about a month or so ago asking them about what the RCA errors in my logs could be meaning for my end users and they could not give me a good answer. This ties it all together since I do have some clients experiencing this exact problem.

  10. We are having this issue and I’m doing my best to follow. In reading is indicates that with Exchange Server 2010 Service Pack 1, it increases the maximum value for RCAMaxConcurrency to 2147483647. If this is true, shouldn’t this problem go away by itself or am I still missing something? I have not done the ‘outlook.exe /resetnavpane’ switch. My users only have maybe 3 calendars open at any one time.

    • Hello,
      Read the KB carefully: Important The original release of Exchange Server 2010 allows a maximum parameter value for RCAMaxConcurrency of 100. Exchange Server 2010 Service Pack 1 increases the maximum value for RCAMaxConcurrency to 2147483647. The maximum value has increased yes, but you should check what is is set at on your servers, the default is 20 I believe. Outlook.exe /resetnavpane was needed in many cases, so do test it to see if it helps. Also don’t forget about possible issues with add-ins and do check out the link to Kovai’s blog on technet for even more options.

      Good luck and let us know how it turns out.

  11. I am having the same problem, contacted MS about the issue and they said it was a known issue and ugprade to office 2007 or 2010, I have tried your fix here but am still having issues, maybe i need to wait for ad to replicate.

    Also tech from microsoft is supposed to be revewing if the Update Rollup 3-v3 for Exchange Server 2010 Service Pack 1 fixes this issue, he recomended we aply it, do you know if this woudl fix the issue?

    • Hello, There are 2 fixes in the blog I needed them both: setting the ThrottlingPolicy AND the outlook.exe /resetnavpane on very client. There are other causes in the i blog and its comments to and there is also the blog post I link to by the engineer I worked with a very good overview. Please do check them all systematically.

      Exchange 2010 SP1 RU3 was re-released due to a bug with Blackberry devices. The change it contains in regards to Outlook 2003 is the return of UDP notification support which made it annoying to impossible to use OL2003 on Terminal services as you can’t use cached mode there. To by knowledge it is not related to this bug. More information on RU 3 is on my blog.

      Good luck & success!

  12. Excuse me if this question is silly, how do we set the throttling policy for the shared calendars that reside on public folders. I am getting lot of tickets for this as users trying to access shared calendars on public folder are getting the “server unavailable error”. I am still on Exchange 2010 RTM version.

    • Not a silly question at all. I haven’t had this issue with shared calenders in public folders but you assign either a (the) default policy or you create your own and assign this on a per user basis. It’s not like there is a mail database of public folder database specific policy. It’s a client policy. Now there is a difference in that PF are accessed on the mailbox server directly that hosts them and not via the CAS role but wether this impacts in anywhay the throttling policy I don’t really know. Have you tried for a couple of users and did you cobine it with the /resetnavpane option with outlook.exe?
      Perhaps a good question for the Exchange newsgroups. Also see all other possible cause in the blog post’s linkt to Kovai’s blog on this eror. Good luck!

      • I have actually created a new policy with RCAMaxConcurrency value as 50 and assigned to couple of users who reported the problem, 1 out of 4 users came back saying problem not resolved and all the users are using outlook 2003. If the calendar would have been of a shared mailbox i could have applied the throttling policy on that shared mailbox, but i am not sure how to go about PFs.

  13. Worked great! It has been driving users crazy for 2 weeks! I suspected it had something to do with a connection limit based our testing of the issue.

    Thanks for posting!

  14. Pingback: The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action. | dailyadminlife

  15. Also having this issue i have set the max connection to $null. Did not fix the problem, before that i had up the connections to 75 still had the same problem. I’m going to try the resetnavpane when i’m back in the office. I really hope that does the trick. Thanks for the extra info.

  16. Pingback: Exchange 2010 SP1 and Outlook 2003: Shared Calendars and Blank E-mail Issues |

  17. First off, thanks for this detailed article. After walking through all of the mentioned steps and applying SP1 for Exchange 2010, I still experienced the same problems. In the end it appeared that the ThrottlingPolicy was never applied to mailboxes and there were custom values enabled in the registry. I wrote a blogpost explaining how to check on misconfigured throttling policies and regkeys at

  18. Awesome we have been looking at this problem for weeks now so was very handy to find this post.
    Thank you so much for sharing 🙂

  19. Thank you for sharing this. We had the symptom appeared following a network outage and all secretaries using outlook 2003 could not open their mailboxes. The event ID 2915 recorded indicating theThrottling Policy and the threshold.
    “Process Microsoft.Exchange.RpcClientAccess.Service.exe (PID=10140). User ‘Sid~domainusername~RCA~false’ has gone over budget ‘208’ times for component ‘RCA’ within a one minute period. Info: ‘Policy:DefaultThrottlingPolicy, Parts:MaxConcurrency:207;’. Threshold value: ‘100’.
    New Throttling Policy with a max value of 200 for RCAMaxConcurrency and an urgent replication fixed it.

  20. Thanks for sharing this long post wth us. This problem accured by one of my customers too.

    Exchange 2003 > Exchange 2010 with outlook 2003. Did al lot of rescheard on the net on what can go wrong but this didn’t pop-up. I`m goging to try this one next week. I will let you know what my results are.

    thanks again for sharing this one.


  21. As promised I should inform you about my progress. I was been able to perform this action earlier.
    I’d created the policy with success. After that I change the RCAMax to 100
    First I checked the ApplicationID connections. Before I applied the policy to a user. And this user had around 25 connections. Not good. I’ve seen the error messages (only in Dutch 🙂 )
    Both CI and MSEchangeRPC (server side)
    Get-LogonStatistics –Identity user | fl applicationid
    ApplicationId : Client=CI
    ApplicationId : Client=MSExchangeRPC

    Did the service restart and AD replication
    • force AD replication
    • throttling service restart
    • RPCClientAccess service restart
    The user added al the calendars (around 25). After that I have asked the user to open and close all the calendars. The user did this 3 times (open and close). No errors :-D. Before the change it happened after opening 5 calendars.

    The problem looks to be resolved.
    Thanks you!

  22. I just wanted to say thank you for this. Step by step and easy to follow, in the end your comment about outlook.exe /resetnavpane fixed this issue for us. We are 2003 Exchange/Outlook moving to 2010 Exchange, funny how something so simple made such a difference. Thanks again you saved a lot of headache.

  23. Thanks for the info. We recently migrated from Exchange 2007 to 2010. Our problem was a little different. Our users were having issues with calendars as well as public folders.

    The affected users would get the messages “Connection to Exchange has been lost” and then “Your connection has been restored.”

    I created a Throttling policy based on your steps and applied the policy to one of the affected users. Restarted the RPC Client Access service on our 2 CAS and 2 MBX servers. Opened Outlook with the /resetnavpane switch and tada! the problem was solved. =)

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