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DJLC - thanks, but to keep us on target I believe this to be an issue specific to OS X Lion and Mail 5...
 
New to this forum and I know this thread is old, but I had a problem similar to the OP's. Gmail account sending and receiving emails but Google Apps account only receiving, not sending, in Mail 5.0 w/ OSX Lion. Pulled up Preferences -> Accounts -> Edit SMTP Server List menu for the Google Apps account and went to the Advanced tab, changed Authentication to Password and logged in with my Google Apps username/password. Seems to have solved my issue. Got this from mrapplegate's suggested settings for MobileMe accounts.
 
Port 25 is not used for SSL.


Incoming Mail (IMAP) Server - requires SSL: imap.gmail.com
Use SSL: Yes
Port: 993
Outgoing Mail (SMTP) Server - requires TLS: smtp.gmail.com (use authentication)
Use Authentication: Yes
Use STARTTLS: Yes (some clients call this SSL)
Port: 465 or 587
Account Name: your full email address (including @gmail.com) Google Apps users, please enter username@your_domain.com
Email Address: your full Gmail email address (username@gmail.com) Google Apps users, please enter username@your_domain.com
Password: your Gmail password


Apple has nothing to do with Gmails servers, so no need to check with them unless you use me.com.

In Mail 6.0 this step just hangs when adding the Incoming Mail Server?
 
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