Some ISP don't allow SMTP at all. FWIW, if you are hosting on Amazon for example, you can't run mail server on port 25 from an EC2 until you submit a support ticket.
True, but this is a small price to pay to setup a mail server in EC2. My reverse DNS request was answered in 48 hours. It took me far longer than that to configure the server. Most spammers won't bother with rdns, dkim, and spf, which is why you should. It's at least a first level signal that you intend to run a clean mail server and that others should trust you.
