Can't Send Mail in Snow Leopard? Try This
Upgraded my Macbook from Leopard to Snow Leopard on Saturday, and all seemed to be well until I couldn’t send emails from Mail.app this evening. A google search revealed that others seemed to be having the same problem, but one of the fixes outlined on the Apple forums seems to have done the trick.
- Close Mail.app if it is open.
 - Open Finder, and go to your user folder, then Library (that is, the Library folder for your user, not the system-wide one).
 - Rename the “Mail” folder to something else.
 - Re-open Mail.app. It should tell you that it needs to import mailboxes. DO NOT click Continue just yet!
 - Go back to the Finder window, and rename the Mail folder back to “Mail” again.
 - Switch back to Mail.app, and now click Continue.
 - Mail should reimport your messages, and you should now be able to send email again just fine!
 
Edie
Edie