Email "from" is not using Default account
It's a known bug introduced in Mountain Lion and was submitted to Apple by several people.
Mail ignores the Send new messages from: Account of selected mailbox setting.

See for example:


https://discussions.apple.com/thread/4148125?start=0&amp;tstart=0
http://forums.macrumors.com/showthread.php?t=1414206
http://mjtsai.com/blog/2012/08/03/mail-mountain-lion-reply-account/


I submitted the bug and is now marked as duplicate (but Apple's reporting tool does not allow you to see the duplicated entry).