Don’t feel too bad. Back in the day, it seemed like every other edit to sendmail.mc would result in an open relay. I don’t miss Sendmail. :)

I would also encourage you to start monitoring your mailq. For a little home server, if you have more than 10 messages in there, you should probably have an alarm going off.

You might also do this for other things like web servers that might send out a contact form email or things like that.

Most consumer ISPs these days block outgoing 25, and it’s been that way since the late 90s. Third party mail providers generally ask you to use 465 or 587 for that reason.