[Melkman]

Socat doesn't support STARTTLS. If you want to debug SMTP submission "openssl s_client -starttls smtp -connect server:587" is awesome. Just take care not to use upper case "R" or "Q". Man, why did they implement rekeying and quit in such a bothersome way. Just use recipient and quit instead of RECIPIENT and QUIT.

[jwilk]

Since OpenSSL 1.1.0, you can turn it off with -nocommands:

https://github.com/openssl/openssl/commit/6ba8a5b77af5792b

[hddqsb]

See my warning regarding s_client and security: https://news.ycombinator.com/item?id=36179850#36186559