If you’ve got an ISP that forces all outbound e-mail to go via their servers, the solution to this is called SMTP smart hosting, where one SMTP server uses another SMTP server as a relay or proxy, essentially acting like a Mail User Agent (client) than a Mail Transfer Agent (server). If running FreeBSD, the default mail server will be sendmail and it’s a little unclear how to set this up.

For me, I just decided to really start from scratch with a custom config file with the SMART_HOST setting. Here’s the file I created, saving it as /etc/mail/custom.mc:

divert(0)
VERSIONID(`$FreeBSD: releng/12.2/etc/sendmail/freebsd.mc 363465 2020-07-24 00:22:33Z gshapiro $')
OSTYPE(freebsd5)
FEATURE(access_db, `hash -o -T<TMPF> /etc/mail/access')
FEATURE(mailertable, hash -o /etc/mail/mailertable)
FEATURE(virtusertable, hash -o /etc/mail/virtusertable)
DOMAIN(generic)
DAEMON_OPTIONS(`Name=IPv4, Family=inet')
DAEMON_OPTIONS(`Name=IPv6, Family=inet6, Modifiers=O')
MASQUERADE_AS(`freebsd.mydomain.com)
FEATURE(`masquerade_envelope')
define(`SMART_HOST', `smtp.mydomain.com')
MAILER(local)
MAILER(smtp)

Then ran a few commands to build the config file using m4 and restart sendmail:

cd /etc/mail
cp sendmail.cf sendmail.cf.bak
m4 /usr/share/sendmail/cf/m4/cf.m4 custom.mc > sendmail.cf
touch local-host-names
/etc/rc.d/sendmail restart

Since I’m sending the e-mails via Python, I used this test script:

import smtplib

subject = "Test"
sender = "me@freebsd.mydomain.com"
recipient = "me@gmail.com"
smtp_host = "127.0.0.1"

message = f"From: {sender}\nTo: {recipient}\nSubject: {subject}\n\n"

try:
     server = smtplib.SMTP(smtp_host, port=25)
     server.ehlo()
     server.sendmail(sender, recipient, message)
     server.quit()
except Exception as e:
     quit(e)