Unexpected behavior of MS Outlook 2013 IMAP client when the IMAP Server is migrated to a new machine

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Recently during an outage we encountered on one of our cloud servers, we learnt something about the behavior of MS Outlook 2013 IMAP client, which is worth sharing via this post.

To verify our observation of this behavior during the outage phase, we conducted tests on MS Outlook 2013 and other IMAP clients in our labs, which are documented below with conclusions on how to handle such backend changes to minimise the impact on end users.


A Mail server (bare metal or virtual) having Connect Xf  is running the IMAP service. Using a variety of mail clients such as Thunderbird, MS Outlook, Baya, mobile, users connect to the Mail server over IMAP to view their mail boxes. Typically when the users connect for the first time, the IMAP client synchronises the mailbox from the server to the client and then for every connect, it maintains the mailbox in sync. Read more about how IMAP works

Depicting an IMAP server and IMAP clients

Now if for any reason, the mail server is changed (rebuilt on another machine or even on the same machine with the same IP address and DNS host names and with the same data restored), we have observed that some IMAP clients “misbehave” as if they were a little confused by what happened back there. The unexpected behavior includes attempting to rsync the entire mailbox and not syncing any mail from the mailbox after the server change, etc.

Depicting the rebuild IMAP server and IMAP clients

Depicting the rebuild IMAP server and IMAP clients

Observed Behavior

We have documented the behavior as observed by us below and have also suggested remedial action to be taken on the client.

Lab conditions for the test:

  • During the rebuild procedure, the server was totally inaccessible to the clients. This means that if the clients were running, they would be attempting to connect to the IMAP server but would not be able to locate the server (via IP or DNS name).
  • Once the server is rebuilt and is back online, the IMAP client which is continuously attempting to connect to the server,  will now find it online. The behavior in the IMAP clients is observed and documented at this stage.
  • The email clients are configured as per steps given here.


Thunderbird on Windows, exhibited this behavior, but was not a serious issue. It was easily resolved by simply restarting the Thunderbird client.However, we were not so lucky with MS Outlook 2013, which was the only client that got badly shaken up when the IMAP server was changed and needed a full reconfigure and full re-sync of the entire mailbox, which can take a long time depending on the size of the mailbox and also would inadvertently load the server during this process. We found that it is possible to avoid this if the MS Outlook 2013 client is configured (in advance) with the Root folder option set.

Mind you that this is not required for MS Outlook 2010.

References: IMAP issues affecting Outlook 2013 and Office 365 functionality

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