You had 5550 errors a couple of years ago. I don't know if this old post is useful or not. I think the solution was to trash a problematic message, but I can't really tell.
I can't send email either. I can receive it though. I also have a MacPro and G5. The email settings are the same on all computers but the G4 running OS9 is the only one that won't let me send. I am getting the same error message.