How can I make Outlook 365 (the desktop app) work reliably with IMAP?

Question

I have dozens of cases of Outlook + IMAP where I see this behavior:

Initially Outlook works fine but after some weeks it stops syncing. When Outlook stops syncing the same account keeps syncing just fine to other clients (e.g. Thunderbird or various Android mail clients). Most of the time I can fix Outlook by deleting and recreating the account or reinstalling Outlook or after some similar "erase-and-recreate-everything" technique. But in a few weeks it will fail again.

During the last 3 years I've seen this happening on three completely different IMAP servers and on at least a dozen of different PCs. Today I decided to google about it and I found plenty of cases with exactly the same [1] or similar symptoms [2]. The reason I'm posting here is that no one offers a solution (except from either ditching Outlook or switching from IMAP to exchange/MAPI).

I hope my google-fu has betrayed me and there is some hidden setting or workaround somewhere.

But if the combination of Outlook+IMAP has proven similarly unreliable for you too, then please +1 the relevant comment below this question (or share your experience if you have something more to add). That will not solve my problem but at least I will not loose more time trying to fit a square protocol in a round client. That's also very valuable for me or anyone else that will face this problem.

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[1] Same symptoms

2017 https://community.spiceworks.com/topic/2078013-is-there-a-way-to-make-outlook-2016-reliable-with-imap

Is there a way to make Outlook 2016 reliable with IMAP? I set it up properly and it works fine - initially. Then, sometime after only a couple weeks, I get calls […] I've been able to move some to Thunderbird. Once that happens calls stop. It just works. […] About a third of the time I just delete the account and create a new one using the exact same settings. Then the user is good until the next time […]

[2] Similar symptoms or people that say that Outlook doesn't play well with IMAP

2014 https://superuser.com/a/703199/257859

[…] If you delete your IMAP account and re-add the account, it will resync correctly. Upon reading forums, I've noticed that Outlook starts getting unsynced around 1-2 months of usage

2014? https://bccs.net.au/2015/08/16/sync-issues-with-imap-and-outlook-2013/

With the release of Outlook 2013, Microsoft changed how they had implemented IMAP support within the Outlook program, which has resulted in countless issues being experienced, troubleshooted, complained about and not resolved. These issues seem to affect IMAP users using Gmail, Outlook.com and ISP email services. […] Yes, there is an inherent problem with IMAP and Outlook 2013. We feel your pain and frustration.

2011 https://arstechnica.com/civis/viewtopic.php?f=15&t=1140941

Why does MS keep pretending Outlook has IMAP support? […] Is there some bit of magic dust that can make Outlook stop sucking so badly as an IMAP client? [REPLY: No]

2010 https://news.ycombinator.com/item?id=1991426

Outlook support for IMAP is extremely poor. It always has been. I'm truly not one for conspiracy theories, but the fact that IMAP is barely compatible with Outlook certainly can't be hurting Exchange server sales. […] It doesn't help that IMAP servers are banned inside Microsoft, so absolutely nobody uses it (you could still connect to external IMAP servers from inside MS, but I don't know anyone who does it)

Answer 1

What's the version of your Outlook 365? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.

Based on my test in Outlook 365(version is as shown in the below image), the emails of IMAP account can be synced successfully. Do you configure your IMAP account with other accounts in a profile? If so, it's suggested that you could configure it separately in a profile to check if there're any differences.

On the other hand, have you tried to manually click the "Send/Receive All Folders" button to see if the emails can be synced?

If so, through my research, this problem is related to the sending and receiving frequency. In order to resolve this issue ,please try the following steps:

1.Click the "Send/Receive" tab > Send/Receive Groups ▾> Define Send/Receive Groups > New… > edit;

2.select "Send mail items" and "Get folder unread count for subscribed folders";

3.Select "Use the custom behavior defined below" under "Receive mail items" > right click the mailbox and choose "check all subfolders";

4.Select you another IMAP account, and do the same settings;

5.In order to receive new mail in a short time, check the second "Schedule an automatic send/receive every n minutes" option and modify the sync frequency time to 1 minute;

6.Then close and restart your outlook. Check if there is any improvement.

In addition, I found a thread mentioned that if you configure two IMAP account in one profile, Outlook does not sync emails until you open "inbox". If it's your case, please refer to JeffYang-MSFT's reply. For more information, please refer to this thread: outlook 2013 does not sync emails until you open "inbox".