Hi,
I had the same issue today (after updating to to 2018.1.4) where mail activities was latest version but kept getting that error.
What I found was that MimeKit version was 1.2.7.0 and an update was available ( you need to uncheck Filter Activities).
After update to version 2.0.3.0 (which was latest available from Package Manager), the error stopped and SMTP send resumed working.
Good luck!