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Office 365 P1 IMAP migration painfully slow

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We finally cut over our legacy on-premises Exchange 2003 (SBS 2003) this past Sunday after 2 weeks of working out issues with duplicate domains in FOPE from an old account.


It was then time to begin the mailbox migration, using IMAP since it was readily available as we had never configured RPC over HTTP. We understood that it might be slower to import mailbox content in this manner, but this is rediculously slow.


We started a migration batch on Sunday of all mailboxes with 3 concurrent connections allowed at a time. Our IMAP server has no connection limits set (per migration best practice). Office 365 was unabe to complete any of the migrations successfuly with this setup; they all failed with errors regarding delays on the remote server. I  never had any issues accessing the IMAP service on the SBS server during this time.


So I restarted a test migration batch of the two smallest mailbox to run with only 1 concurrent connection. I am able to sync one of the smaller mailboxes (942mb 5000 items) from the IMAP server (5mbit upstream) to my home internet connection in about 30 minutes. Office 365 took over 4 hours to sync a mailbox half that size (500mb, 150 items).


Ok, maybe that was because I was running during daytime hours. So i re-queued up the full migration again and now it's been running the import of one of our larger mailboxes (9gb, 32000 items) and it has been running cummulatively for over 3 days and has only imported 15,578 items. This is rediculous.


I also notice often when accessing the Office 365 Outlook Admin pages I often get errors about my session exceeding bandwidth throttles and I have to wait a few seconds to retry the operation, or occassionally just basic errors about a service being unavailable. What gives?

All Replies
  • Hi Peter,

    I understand that it takes a while to perform IMAP migration. When accessing Office 365 Outlook Web App Admin pages, the error “session exceeding bandwidth throttles” occurred. When retried after a few seconds, the error message about a service being unavailable occurred.

    Peter, with IMAP migration, after the initial synchronization of mailbox data occurs, the process can take a good deal of time to complete. For a large number of users with large mailboxes this process can take up to a few days. This will be an issue if you are migrating a large number of mailboxes in one batch. Before the data is migrated all of the users will be provisioned with a mailbox allowing them access to their mailboxes in Exchange Online.

    To make sure the migration more smoothly, you can click Complete to finish the migration, and then inform users ahead of time about what to expect when there mailboxes are migrated, also if possible migrate the mailboxes during non-peak times such as over the weekend.

    If the migration batch is hung at the data migration process, please check the disk/network usage of mail server in your on-premise domain with performance counter first. And then, you can monitor migration process during the IMAP migration with following cmdlets.


    Please Note: Before running these cmdlets, you should connect the windows PowerShell to Exchange Online first.

    By the way, if these are few users with mailbox, you can also migrate emails for user by exporting/importing .pst file with Outlook. For detailed steps, please refer to following article.

    If any error message appears, please capture a screenshot and post it in the forum for further research.

    Monica Tong

  • All the mailboxes were provisioned prior to starting the migration.

    I had already run the commandlets before posting here and there were no errors.

    In the past 11 hours the single mailbox that is still in process has not synced any additonal items but there remains no errors on my end or on Office 365 end.

    I guess I need to cancel and restart the process yet again.

  • Hi Peter,

    Thank you for your updated reply.

    I understand that you have run the commands and there weren’t errors. You will click Cancel and restart the process again.

    Peter, if there is any progress about this issue or you need further assistance, please post it in the forum.

    Monica Tong

  • We have determined that IMAP migration is simply not feasible based on Office 365's limited ability to handle the mailboxes sizes we currently have. I am going to test RPC over HTTP this weekend if possible, otherwise we will simple resort to manual PST export & import.

    This migration experience has significantly reduced our opinion of Exchange Online and are now second guessing our plans to migrate our Google Apps customers to Office 365 as the profit margin is completely lost on the implementation overhead (Google Apps setup takes us less than 1 day per customer, migration to Office 365 has taken over 1 month of time and at least 25 man-hours and is still incomplete).

    We are a MS SBSC Partner by the way.

    1 out of 1 people found this post helpful.

  • Hi Peter,

    If the migration still doesn’t finish properly when testing with another method, please provide detailed description about the situation for further research.

    Monica Tong

  • Manually importing from PST worked without issue - this was the very thing we were trying to avoid but seeing as we've already invested so much time in trying to run an IMAP migration, what would a couple extra hours hurt? It sure saved us from having to waste another dozen hours fighting with Exhcnage Online's non-functional IMAP importer. This has been one of the most disappointing experience ever and my technicians are now telling me "I told you so - should have stuck with Google Apps." I don't think we'll be migrating any more clients over to Office 365 as planned; at this point we're just performing damage control with this particular client as Microsoft has made us look like complete fools, selling a service as advertised only to not deliver on any of it.

    2 out of 2 people found this post helpful.

  • I'm mean seriously, just using the Exchange Online web interface is painful - nothing but bandwidth throttling messages and service unavailable errors. You guys are Microsoft, get some load balancing for heaven's sake.

    1 out of 1 people found this post helpful.