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.