Check for blacklists
You should mention how to check the intended IP of your mailserver isn't on a load of internet blacklists that mean trying to send mail to anyone at gmail, hotmail etc. will be permanently rejected until you 'fix your spam problem'.
Trying to run a mail server from a consumer IP block, even if fixed, is a recipe in frustration otherwise.
And it's probably a good idea to do this before you buy the hardware, and start configuring Dovecot and Postfix.