We've got the same problem here, with Eset Endpoint Security 11.1.2039.2 and Outlook 2016/2019/2021.
It seems like that it occurs when the message is sent to the Outbox, not when it's sent via SMTP. I'm saying this because it occurred for us in offline mode as well. Disabling the Add-In via policies work but it feels like a bit of a nuclear option.
I submitted a ticket on the Support portal, but i hope this will help you pinpoint the issue.
An Outlook plug-in with a fix is being tested and the fix should be included in the next version of Endpoint if users confirm that it has fixed the issue.
Arthur CROSNIER received kudos from Bragard in Problem with Eset Endpoint Security 11.1.2039.2 and Outlook 2019
