I know that residential ISPs block outgoing port 25 because of the spam issue. Is it common for them to block it incoming as well? You don't really need outgoing port 25 to run your own mail server.
Directly, yes, but not via a smart host, which is how most SOHO mailservers (where used) are configured.
It's not quite a full-fledged, full-privilege system, but it continues to provide major benefits over Webmail or client-based (POPS, IMAPS) configurations.