"invalid first reply" error after Windows update

I’m running version 6.0.23421.0 and things have been OK since I installed it several months ago, but yesterday Windows (10) did an update and now I keep getting “invalid first reply” errors.  How do I fix this? 

Hi Mark,
could you click the third tab titled Log in the same window as the error message and copy the content for me here next time this error pops up?


I will.  The problem seems to have cleared itself (after about 20 hours!), and may have been with my mail server.  But the error message was peculiar, and frustratingly unhelpful.