It’s been for long time that tool for “send email” is semi broken
I spend like 1h to trying to figure out why I cannot send email via mikrotik OS, been adding certs, tweaking ports, checking log message on mail server…
and end up testing the email capability with CLI with success.
Email setting windows has STARTTLS option: NO, YES, TLS ONLY, CLI has TLS parametr that has = YES, NO, STARTTLS, the Send mail window only has: TLS = YES, NO
So I was unable to test smtp settings with Send mail window 
because my mail server only allow for STARTTLS connection - yes, I could change that but thanks to that I nottice that those options are incosistance.