Having a mail tansfer agent, waiting for a pipe input right there in our $SHELL is a blessing.
Luckily for us, OpenBSD comes with a nifty thing called smtpd, which can be
configured via smptd.conf(5).
Let's make a file, /etc/mail/secrets as root. And in it add your relay,
which is a fancy term for what you wanna call the service, you wanna use to send your mail.
Well, the technical definition is
"SMTP Relays lets you send email messages through your email account using your existing email client". Nuff said!
$ doas cat /etc/mail/secrets
# myrelay relay_username:relay_password
fmail akts@fmail.org:5tr0ngP4ssw0rd
Add a little bit of security to that file by locking down its permissions.
$ doas chown root:_smtpd /etc/mail/secrets
$ doas chmod 640 /etc/mail/secrets
Now bust open /etc/mail/smtpd.conf and change it like so
...
table aliases file:/etc/mail/aliases
table secrets file:/etc/mail/secrets
listen on lo0
action "local" mbox alias
action "relay" relay host smtp+tls://fmail@fmail.org:587 auth
match for local action "local"
match for any action "relay"
...
587 is the default port for SMTP on most mail providers.
Check if the config syntax is OK with $ doas smtpd -n.
Now, do a $ doas rcctl restart smtpd. The SMTP daemon should be working now.
Let's send a mail,
$ mail -s "A Test email from your OpenBSD machine" -r 'akts@fmail.org' ecorp@gmail.com
Take a look at the mail(1) man page, before we get creative.
And before we end the article, do a $ doas tail -f /var/log/maillog to the see the action.