The port change thing, you would do by going into Mail's preferences, picking your .Mac entry from the Accounts icon, and picking the Server Settings... button at the bottom of that window. But, I don't think I'd bother with that now that you are saying some messages do get through.
You might want to bring up Mail's activity viewer (cmd-0 -- that's a zero) and see if any interesting messages come up while you are trying to send. It's hard to tell what is happening right now since your results aren't consistent.