Comment 31 for bug 1899485

coxmatthew, would you mind trying uninstalling the Linux Mint package installed version, and installing the Mozilla-provided binary, just to see if there's a difference? If you can still reproduce the issue with the official Mozilla binaries, that would be useful to know.

You can download the official build here:

https://www.mozilla.org/en-US/thunderbird/all/

If you can still reproduce the problem, then I'll go and set up a VM here with "Linux Mint 17.3 Cinnamon 64-bit (Cinnamon v 2.8.8)" and see if I can reproduce the problem here, as well. Also, you said you cannot reproduce the problem when using a Google/Gmail account over IMAP/SMTP, but you can when using an account through DavMail IMAP/SMTP when DavMail is connected to Office365? I basically want to replicate your environment so that I can reproduce the problem on my own, so I just want to confirm the details.