Add the following to ~homeuser/.nailrc to set the default SMTP relay:

~homeuser/.nailrc

# Remote SDF SMTP relaying (use SDF 'mkvpm' tool to set auth user:pw):
set ssl-verify="ignore"
set smtp-use-starttls
set smtp=mx.sdf.org:587
set smtp-auth=cram-md5
set smtp-auth-user="sdf_user.sdf.org@sdf.org"
set smtp-auth-password="my_password"''

The SDF CA is self-signed but can still be used if desired. Retrieve and store somewhere (ie. under ~/.openssl/) as pem file, then edit ~homeuser/.nailrc like so:

~homeuser/.nailrc

set ssl-ca-file="/home/home_user/.openssl/sdf_self-signed_ca.pem"
set ssl-verify="warn"  # will issue "self-signed CA" warning
...

Account-specific smtp settings are possible; see the Heirloom Mailx documentation.

Heirloom Mailx under Linux/BSD - traditional link (using RCS)