This is where email server admins need to stop and pause. It’s one thing to have an “easy to run” email server, and kudos to Josh for all his hard work. BTW but understanding email concepts is important.
Email servers shouldn’t be run for example on IP address ranges that are allocated by DHCP - this is because even if they don’t change very often (and I’ve had my current Virgin Media cable IP for over a year now), they MAY change.
To that end email reaching Mail exchangers SHOULD NEVER come from one of these IP addresses.
This has prompted the creation of a number of Policy Blacklists which include as many known DHCP allocated IP ranges as possible. Two of the most well known are:
Note that if you are on the PBL you will also be included in Spamhaus’ Zen blacklist which is a master list of all the different zones that they have, this enables mail exchanger admins to make one single query that covers all Spamhaus’ blacklists.
Email programs aren’t affected as most mail clients talk to dedicated email servers that authenticate the connection via different means before accepting the email.
Edit - Indeed even MIAB treats mail coming on on port 25 differently to mail coming in on port 587 for precisely this reason, in the former it’s acting as a mail exchanger, in the latter it’s acting as a submission server.