It is probably true that my isp blocks port 25; it will be easy to determine for sure. If so, are there any solutions you can suggest?
There is no good way around this. About the only thing you could do is have another account store the incoming mail somewhere and you your local mail server do a "pull" of sorts using something like "fetchmail" or something. ...or petition your ISP to unblock port 25 (unlikely) or see if they have a "business class" type plan that will give you a static IP and hopefully unblocked port 25.
Look into rollernet.us. They offer a free smtp redirection service where they are listed as your MX, and accept connections for your domain on port 25, and forward the message back out to your smtp server on any port you specify. See http://rollernet.us/services.php?smtp