I've verified that a few bugs from Lion have been fixed in Mail. But I keep finding more new bugs, some of them critical:

New to Mountain Lion: Reply to a message in one of your local folders and your FROM address will *not* be automatically set to the recipient address of that original message (what has always happened). Instead it will be set to the primary address of the email account that appears first in your list of inboxes in your mail window's source list.

New to Mountain Lion: Compose a new message while a message in local folder is selected and the FROM address will be automatically set to the first address of the first account listed in the source list. It has always used the recipient of the message that was currently select (if it was one of your defined addresses) before. This one I don't mind too much.

From at least Lion 10.7.4: Creating a NEW message will not use the selected FROM address if that address is not the first address listed for a particular account. Example, an account with 4 email addresses listed, cannot send from addresses 2-4. It's always been possible to list additional email addresses (like aliases) with every account. Now it's impossible to use them to send mail. The copy of the message in SENT MAIL shows up correctly with the selected email address as FROM, however the mail actually gets sent out as from your first email address listed in the drop-down menu of all your email addresses. *

* The drop-down list of email addresses are sorted in the order of INBOXES listed in the mail window's source list, but if a change is made to the inbox order, the email list is only updated after quitting and restarting Mail.


These three are very dangerous. With the first two it's possible to catch the issue before sending out the email, with the third it's not. It's impossible to use these other email addresses in Mountain Lion to compose new messages.