Minor nitpicks: a) you need a plain-text aka telnet client for this b) if you receive a valid, human-readable text then it means what you are not on TLS for s

Minor nitpicks: a) you need a plain-text aka telnet client for this b) if you receive a valid, human-readable text then it means what you are not on TLS for sure c) if B succeeds that doesn't means what that SMTP server support STARTTLS, you should check options and try to initite it , eg: 220 smtp.fastmail.com ESMTP ready -> EHLO just.testing.things 250-smtp.fastmail.com 250-PIPELINING 250-SIZE 71000000 250-ENHANCEDSTATUSCODES 250-8BITMIME !! 250 STARTTLS -> STARTTLS 220 2.0.0 Start TLS