Classic Outlook 2021 seems to be sending expunge requests to IMAP servers sequentially, instead of batching them

Question

We host a Linux based IMAP server. Our users use mostly Thunderbird, Apple Mail or different versions of Microsoft Outlook (Windows) as email clients to connect to the IMAP server. For Microsoft Outlook email clients, we have some running 2016, some 1019 and some 2021 (under the umbrella of MS Office for business).

One of the MS Outlook 2021 users, selected a year worth of emails and requested them to be purged from the server and Outlook 2021 seems to be doing individual requests for the deletions. The user is complaining about the performance of his email client. From the server logs, I can confirm that it seems to be requesting to delete one at a time. Individual requests seem to hog the resources of Outlook and he can't see new messages for 15 minutes or so and the deletions are still going on.

I have confirmed that one Outlook 2016 client sends batch requests for similar deletions (from the server logs also). I've compared all the settings between the 2016 and the 2021 emails clients and they're the same.

Some users in other forums have mentioned that the new behavior could be to keep a more consistent local database.

By the way, I've tried to to generate transport logs in both 2016 and 2021 and haven't been able to. New logs are created in the appropriate folder, but it only adds ETL files and other .log files that have very generic information. I spent a good couple of hours and gave up on it. This is why I have to rely on the server logs.

Did the interaction with IMAP servers change for newer versions?

Answer 1

Hi @RG007

Good day and I appreciate the clear explanation of your concern.   

Currently, Microsoft has not published any official documentation regarding how IMAP delete or expunge operations are processed, and there is no available setting in Outlook to control this behavior. 

As far as I know, IMAP message deletion is typically a two-step process: messages are first marked for deletion and then permanently removed during an expunge operation. However, whether these operations are issued in batches or processed individually is determined by the client implementation, not by configuration. 

Additionally, Outlook 2021 belongs to the “perpetual” (one-time purchase) release model, where the feature set and overall behavior are effectively fixed at the time of release. After release, Microsoft primarily provides security updates and limited fixes, but does not introduce significant functional or performance changes. As a result, differences in behavior (such as IMAP processing patterns) are typically not adjusted in these versions. 

Since your environment uses a Linux-based IMAP server (non-Exchange backend), Outlook relies entirely on standard IMAP synchronization. In this scenario, large operations, such as bulk deletions, can result in increased round trips to the server, higher client resource usage, and delayed updates in the Outlook user interface. 

As a practical workaround, you could consider performing deletions in smaller batches or avoiding large bulk deletions directly from Outlook IMAP profiles to avoid this behavior. 

I hope this helps. 

If you have any additional concerns, feel free to comment below. I would be more than happy to assist. 

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