A few years ago, I sent up a Ubuntu server (mail/ftp/web) pretty much following the instructions here: https://www.exratione.com/2014/05/a-mailserver-on-ubuntu-1404-postfix-dovecot-mysql/ Of course the instruction posted there (at the time) were for Ubuntu 12.04 but it's essentially the same. It worked fine I had no problems, and there were no issues. That is until my ISP (Teksavvy) started blocking my ability to send mail. I spoke with them and apparently I have to use out.teksavvy.com on port 1025. Prior to this I did not relay mail through them. I note a relayhost option in the Postfix main.cf file. If I change it to relayhost = out.teksavvy.com:1025 every single message I send out (from myserver.com domain) bounces back as unauthorized with the 550 error. If I change the relayhost option to relayhost = smtp.teksavvy.com:25 I can send mail from my own domain, but I can also send mail from virtually anyone as it effectively turns my mailserver into an open relay. (I managed to successfully send mail from email@example.com to other valid email addresses outside my domain). They claim it's to "cut down on smappners" but if I open up my server and use the relayhost option then it pretty much guarantees a melee (pardon the pun). Since I'm being forced to route mail through my ISP's server now (from my own mailserver), what's the best way to do this to ensure I'm not running an open relay?