security/ismtp: Test for SMTP user enumeration, internal spoofing, and relay

SMTP user enumeration (RCPT TO and VRFY), internal spoofing, and open
relay. A tool that tests for all three and comes with great
flexibility. iSMTP does just that, making it much easier to knock that
process out of the way.

WWW: https://github.com/altjx/ipwn/tree/master/iSMTP

PR: 270304