go-check-smtp is a simple drop-in replacement for check_smtp in nagios-plugins distribution written in Go.
I think check_smtp with STARTTLS seems to be broken because check_smtp does not trigger warning or critical even though SMTP server returns 4xx/5xx.
My SMTP server speaks HAProxy's proxy protocol to receive connections from AWS's ELB so I also need check_smtp with proxy protocol support.
Download from releases.
go get -u github.com/nabeken/go-check-smtp
go-check-smtp \ -P \ -S \ -F localhost \ -H 127.0.0.1 \ -p 10025 \ -w 2.0 \ -c 1.0 \ -f email@example.com \ -r firstname.lastname@example.org