>>>>>I am pretty sure now that the problem has to do with the specific Outlook.pst file. On this computer (where I am typing this message) I have the same Office 365 and Outlook installed. And when I would start Outlook (up until 1/2 hour ago) I never had a problem with memory leak.
>>>>>Then now that Avast completely f*cked up my "main" computer, I need to temporarily move files from "broken" computer to this one. I copied the Outlook.pst from the "broken" computer to this one and now when I open Outlook 365 on this computer and click on Reply to any message, the memory leak starts.
>>>>
>>>>Rebuild the cdxes... :)
>>>>
>>>>Really, sounds like some internal tables got whacked. See if that database (what's it internally, JET?) has an equivalent of pack command. Just rebuild it - or create an empty one and import everything from the bad one.
>>>
>>>It appears that MS removed Data File Management (where you could compress/pack PST file in prior versions) from Outlook 365/2013 :(
>>
>>How about https://pcandtablet.com/outlook-2013/1034/how-to-compact-and-reduce-the-size-of-outlook-2013-pst.html ?
>
>Thank you. This compacted the outlook.pst file but it didn't fix the memory leak. Back to looking for solution.

Long shot - you could try the compaction process multiple times.

I think there are PST fix utilities, I don't have any experience with them so whether they can fix your problem is something of a gamble.

Some more things you could try yourself:

- If you know roughly when the problem started, try copying all PST content near that date to a fresh PST #1 file. For example, if problem started May 5th, copy content from, say May 3rd to May 7th. Open the new PST, see if the problem persists. If so, problem is within the copied content. In that case, you can further copy content from new PST #1 to new PST #2 - copy half, then you can see which half of the data has the bad content. You can keep splitting up the content into new PSTs until you zero in on the bad content. Once you've determined the bad content closely enough for your purposes, back up your original PST, then delete the identified bad content.

- If you don't see the problem with the new PST #1, then there's something wrong with the structure of the original PST. You could manually or programmatically copy all content into a fresh PST, see if problem persists in that one