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