Microsoft 365 - Improving the performance of clients configured via IMAP

Email clients that are configured via IMAP will experience a variety of limitations and performance issues. Due to these issues, configuring your client via IMAP is not recommended. However, if you do decide to configure your client via IMAP, the suggestions below may help mitigate some of the performance issues.

Performance issues for clients configured via IMAP

Below is an overview of the biggest issues you will experience if you decide to configure your email client via IMAP.

  • All clients connecting via IMAP will experience data throttling

    • Throttling causes a reduction of speed when downloading or moving a large number of messages
    • Throttling is controlled by Microsoft and cannot be removed
  • If throttling occurs when you are attempting to move mail, you may lose messages.

  • Most IMAP flags are not supported in Office 365. Only the "Read/unread" and "Important" flags are supported

  • If you have a client configured via IMAP, Outlook on the web will show drafts of messages you have already sent in message threads

How to reduce performance issues

The steps below may help mitigate the performance issues you will experience when using a client that is configured via IMAP. Please keep in mind that these steps will not completely alleviate performance issues.

  • To reduce the impact of throttling when connecting via IMAP, do not sync all folders locally if you have a large mailbox.

  • Compact your mailbox frequently, especially if you have a large mailbox.

  • To avoid losing messages when moving mail:

    1. Copy the messages to the desired folder
    2. Verify that the messages have been copied by logging into your account via Outlook on the web. If not, you may need to wait a little longer.
    3. Once you have confirmed all the messages have been copied, go back into you IMAP client and delete the original messages from the source folder.
    4. Close your IMAP client. Some clients do not purge deleted messages until the application is closed.
    5. Verify that the messages have been deleted from the source folder by logging into your account via Outlook on the web. If not, you may need to wait a little longer.

Remember, the best way to avoid the issues described in this document is to configure Outlook client via Exchange protocol.

Keywordsmicrosoft office365 o365 m365 microsoft 365 imap configure configured configuring setup performance performing improve improving moving copying issues known data throttling throttled throttles moving moved copy copying messages flags read unread lost deleted emails missing lose losing important drafts sent compact compacting mailbox sync syncing local   Doc ID49977
OwnerO365 S.GroupMicrosoft 365
Created2015-04-03 13:36:17Updated2023-02-02 09:22:55
SitesDoIT Help Desk, Microsoft 365
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