Here is my bug report: 12025786:
Summary:
Mail is not defaulting the sender correctly for messages that have been moved into another mailbox by a rule.
This is a serious regression from Lion.
Steps to Reproduce:
1. Set up Mail with more than one account, let's call them A and B.
2. Set up a rule so that messages received on account A, are sent to a folder "ON MY MAC" called "A".
3. Set up a rule so that messages received on account B, are sent to a folder "ON MY MAC" called "B".
4. Receive messages on accounts A and B
5. Go to Mail Preferences, Composing, and set Send New Messages From: Account of selected mailbox
6. Reply to the message in mailbox A, observe the default sending account
7. Reply to the message in mailbox B, observe the default sending account
Expected Results:
The default sending account in step 6 should be A, the default sending account in step 7 should be B.
Actual Results:
The default sending account for both will be the same (either A, or B, depending on the order the account are listed someplace, although theories diverge about what place that is: possibly the preferences accounts list, but more likely the inbox list).
Regression:
In Lion and previous, messages remembered where they came from, so that they would default to the correct sender. The new behavior is extremely undesirable. In fact, I can't think of a single case where it would be preferred over the old behavior.
Notes:
This discussion thread gets at various different versions of this same problem:
https://discussions.apple.com/thread/4148125?start=45&tstart=0
As Jerome wrote:
"This is a bad regression, resulting in embarrassment and confusion for power users when messages are sent from the wrong account. Because power users must use rules, there is no good workaround for power users, and should be fixed as soon as possible."
Also, you will see that bug #12013442 may be a duplicate of this bug.