Postfix

If you are running postfix on a residential machine and wish to send through your ISP's mailserver, the settings in main.cf are documented in ​http://www.postfix.org/postconf.5.html under 'smtp_tls_wrappermode':

smtp_tls_security_level = encrypt smtp_tls_wrappermode = yes

This requires cyrus_sasl, and for some ISPs (hello, virginmedia) you will need to use port 465 in your sasl commands.

A few other servers may still reject you mail, but most will not flag it as spam.