After upgrading to 10.8.3 many users have been having problems sending mail. After pulling my hair out trying to troubleshoot Postfix I found a quick solution that worked for me. I use Postfix to run my outgoing mail so this may not work for your situation.
In Mac Mail
1. Select > Preferences > Accounts > Select an account
2. Outgoing Mail Server (SMTP) – Select “Edit SMTP Server List”
3. Hit “+”
4. Enter Server Name “127.0.0.1”
5. Click “Advanced”
6. Choose “Use custom port:” and Enter “25”
Worked for me, and I hope it works for you. Seems leaving it on the “Use default ports” option is bugging out. If you don’t use Postfix, try using your ISP as the outgoing server will probably work (example: comcast.com).