That's what I meant to say, delete the account from Mail.
I was wondering if you used MobileMe to sync Mail in general, all email accounts, but that doesn't matter with the sending part.
"Cannot send mail. The sender address was invalid" usually means that your user info in the SMTP servers is bad or wrong. Delete your SMTP server for that email address, re-add, and re-associate with that account. You usually are better off using the same SMTP server as the email address you're sending from.
Don't try sending the same email, make a new one BTW.