I can receive emails and reply, but can't send them
Is there potentially a firewall setting in Mail that is preventing my message from sending? I checked my with my email provider and their server is functioning (I can set up the account on my iPhone with no problem). I checked with AT&T and my wireless router is functioning. Both are saying it has something to do with my iMac settings. All the Mac help articles point to having the proper settings which I triple checked with my email provider. I'm confident they're accurate - and it works fine when I set up the account from my iPhone.
When I send an email to the new account I can reply with no issue. The error only occurs when I originate an email from this account. Also, when I check the connection doctor it indicates the connection and login to the server was succeful - no issues. There are no hung up activities in the Activity Window either.
Here's a screen shot of the error message:
Very frustrated!!!
Any help or suggestions woudl be greatly appreciated! Thanks
iMac, Mac OS X (10.6.8)