Currently Being ModeratedFeb 2, 2013 1:41 PM (in response to ifay)
Same problem here. I have install Mountain Lion, but this has not fixed the problem.
I found the following fix from Glen Carter, but I'm not able to find OPT key to hold it down ???
It appears that Apple is aware of this problem and is working on a fix for a future update. In the meantime you'll need to do the following to resolve the issue:
1. Quit Mail
2. Hold down OPT and select LIBRARY from the GO menu
3. Open Mail --> V2 --> MailData
4. First, DUPLICATE the "Accounts.plist" so you have a copy that says "Accounts Copy.plist" (Do not edit this one in the event you need to go back to it).
5. Open the original "Accounts.plist" with TextEdit
6. Scroll down until you see the following:
7. WITHOUT changing any other text, change the "25" to "587" so that it looks like this:
8. Close the Accounts.plist and relaunch Mail.
Now try sending an email.