In my GnuPG article about how to attend a PGP signing party I explained how to set up caff using SMTP (Simple Mail Transfer Protocol). However, using a local Mail Transfer Agent (MTA) is the more elegant solution as SMTP is not a properly supported option and discuraged, according to the caff man page or when setting $CONFIG{'mailer-send'}
to any value:
Ben’s Build and B(r)ass Notes
IT Insights and Build Breakdown