SUMMARY

This article is to help resolve or explain issues when an IMAP client is synchronizing a mailbox and it is taking a while to complete, or fails to complete.

DETAIL

New account configuration in email client is slow and you may not see new emails

When an IMAP client first connects to a mailbox, generally it has to download every message from the mailbox on the server. Depending on the size of the mailbox, this can take some time. It can be made slower by any antivirus application on the client computer, as each message will also likely be scanned. During this initial synchronization, some email clients will not show new emails. Wait until all the emails are synced.

Syncing a mailbox is slow in Microsoft Outlook when XLIST IMAP option is enabled

While the XLIST option is useful for users who want to connect Outlook to the correct default folders with those folder names in their native language, it also changes the behaviour of how Outlook syncs to the server.

When Outlook syncs a mailbox, and XLIST is disabled, Outlook will only sync folders that are subscribed to and the default folders, such as Inbox, Deleted Items, etc. When a user clicks the Send/Receive option, again it will only sync those folders and just check for new emails once synced. So syncing is normally quick.

But if XLIST is enabled for the IMAP service, Outlook behaves differently. When a mailbox is first synced, it will get a list of all the folders and begin to sync everything. When a Send/Receive is performed the client will check every folder for the mailbox and  retrieve the list of flags for each message. The larger a mailbox, and the more folders, the longer this process can take. It may also require up to appromately 500 kilobytes of data to be sent from the server to client for every 10,000 messages in the mailbox. If any new emails are found, these are also downloaded. If an antivirus application is installed on the client, it will also likely scan each new email downloaded. Using XLIST prevents the IMAP folder subscription from being used, but you can improve the speed of a Send/Receive by selecting what folders to exclude from the check in Outlook. This is done using the Send/Receive Groups option and editing the groups. Don't check folders you do not use often, or that are not accessed by other email clients. Outlook will check the folder anyway when you click on it.

Syncing a mailbox never ends

If a sync is performed by the email client, but never finishes, it could be that there is antivirus proxy/application that is blocking emails that are being detected as infected. The client IMAP logs would indicate that message content was requested, but never arrived, possibly giving a timeout. On the server, the IMAP Debug log may show a network error indicating the client has disconnected. Try disabling any antivirus application on the client or bypass any proxy to see if the sync can complete.

Messages in a folder all disappear and reappear over time

If messages all go missing from a folder, but then reappear, it indicates that the UIDVALIDITY value for the folder has changed. This is a unique value used by the IMAP protocol that allows a client to know whether there is a change on the server that requires it to get all the messages again. So if the value changes, an email client will appear to remove all the cached emails and download all of them again from the server. If this problem happens with the Inbox of a folder, firstly make sure that there is not a POP Retrieval item configured for the mailbox causing email to be looping. Then check to see if the _index.xml file in the folder on the server (which is the index file for the folder) does not have its creation date changing. If this date changes it means the index has been rebuilt and the client has to download the folder again. Check the Windows event log if it does, to see if there is an error with the index.