Since the publication of “Take Control of Apple Mail in Mountain Lion,” several people have asked me about changes in the way Mail in Mountain Lion handles the From address in outgoing messages. At first I didn’t know what to say, because I hadn’t seen anything surprising in my own testing, but as more and more complaints have poured in (including many in Apple’s discussion forums), it became clear something was screwy. So I set out to investigate. It turns out Mail in Mountain Lion does handle outgoing mail differently than before, and for some users, it can be frustratingly difficult to predict what account Mail will send messages from.

Apple Mail lets you set up as many different accounts as you need, each with its own outgoing mail server and one or more associated email addresses. When you create a new message or reply to an existing message, Mail picks a default From account for you, although you can choose a different account from a pop-up menu in the header area of the message. That’s all entirely appropriate, and has been the case for as long as I can remember.

However, in Mountain Lion, Apple significantly changed the rules by which Mail chooses the default From account for outgoing messages. As a result, if you don’t carefully (and manually) check each outgoing message, you might find that you’re sending messages from an unintended account. This can have serious consequences if, for example, you end up sending work email from a personal account or vice-versa. I didn’t notice this during my first couple of weeks using Mountain Lion because I nearly always send email from the same account, and I don’t lose any sleep if I accidentally send mail from a different address. But for many users, this change is a big deal.

How exactly has the behavior changed? Here’s what I’ve found, based on well over 100 experiments covering as many combinations of variables as I could think of.

In Lion and earlier, the behavior is as follows:

• When replying to messages, Mail always sends replies from the account to which the message was addressed. This is true regardless of your preferences or where the message was stored.

• When composing new messages, Mail uses the account specified in the Send New Messages From pop-up menu on the Composing preference pane (in other words, exactly what the command says). The default choice, Account of Selected Mailbox, means that the account associated with whatever mailbox is currently selected in Mail’s sidebar is the one that should be used for outgoing messages. However, if a local (On My Mac) mailbox is selected, or if no mailbox at all is selected, Mail chooses a From account according to some rule I’ve been unable to nail down, even after many tests. And, if you have a particular message selected, then regardless of where it is, Mail may choose yet another From account — again, without any immediately obvious logic.

In short, although Lion has some indeterminacy with the Account of Selected Mailbox setting, the behavior is mostly predictable — and when it comes to replies, it’s both consistent and logical.

On the other hand, in Mountain Lion, here’s what I’m seeing:

• When replying to messages:

  • For messages stored in a server-based (e.g., IMAP or iCloud) mailbox, Mail uses the account associated with that mailbox — regardless of the account to which the message was sent or your preferences. That makes reasonable sense as far as it goes, and even though it’s different from before, it should yield the same result most of the time. But…

  • For messages stored in a local (On My Mac) mailbox, if Account of Selected Mailbox is chosen in the Send New Messages From pop-up menu on the Composing preference pane (as it is by default), Mail uses… um… some seemingly arbitrarily selected account. As with composing new messages in Lion, I’ve tried dozens of tests, and every time I think I’ve guessed the pattern (such as the topmost account enabled in the Accounts preference pane; iCloud first, then Gmail; the last account you sent mail from, etc.) I find an exception. I truly don’t know; all I can say is that Mail seems to pick the same account consistently as long as your account preferences remain unchanged. On the other hand, if a single account is specified in the Send New Messages From pop-up menu, Mail uses that account — regardless of the account to which the message was sent. (In other words, it treats replies the same way as new messages.)

• When composing new messages, Mail uses the account specified in the Send New Messages From pop-up menu on the Composing preference pane. As in Lion, Account of Selected Mailbox is the default choice, and for server-based mailboxes, it appears to work the same way. But if no mailbox is selected, or if a local (On My Mac) mailbox is selected, Mail uses the account whose Inbox appears first (under the unified Inbox in Mail’s sidebar); you can drag Inboxes up or down in this list to change their order.

To illustrate why this might be a serious problem, consider a user who has three different POP accounts — say, a home account, a work account, and a family account. Messages from all those accounts are filed into various local mailboxes, perhaps automatically by rules. When it comes time to reply to one of these messages, the user should be able to count on the fact that the reply will be sent from the same account that received it, without further intervention. That used to be the case, but it isn’t now. So outgoing messages may be missed or discarded because they come from the wrong account, messages can cross between business and personal domains, and so on.

I have no idea why Apple made this change, whether it was intentional, why it wasn’t documented, or whether it might be changed back in the future. Because the change might have been deliberate, and might have happened for some good reason that I simply can’t discern, I hesitate to label it as a bug. But in any case the change requires more thought and effort for users, and on the whole I think it was a pretty bad move. If you think so too, choose Mail > Provide Mail Feedback and share your thoughts on the matter with Apple. We can only hope Apple changes this in an update to OS X.

In the meantime, I’m sorry to say I know of no way to return Mail to its pre-Mountain Lion behavior. Choosing a single account from the Account of Selected Mailbox pop-up menu will keep Mail’s behavior fairly consistent, although that may not suit your needs. Likewise, using only server-based mailboxes, if you can do so, avoids the most serious part of the problem. Apart from those admittedly weak suggestions, I can’t offer any advice other than to be aware of the change and remain vigilant when sending messages.

One final tip, courtesy of Christopher Stone on the TidBITS Talk mailing list: In Mountain Lion, you can assign a keyboard shortcut to change the From account of the message you’re currently composing, which may be slightly easier than using a mouse or trackpad to choose the account from the From pop-up menu. To assign a keyboard shortcut, open the Keyboard pane of System Preferences, click Keyboard Shortcuts, and then select Application Shortcuts. Click the + (plus) button and select Mail from the Application pop-up menu. In the Menu Title field, type the exact name and email address as they appear in the From pop-up menu — for example:

Joe Kissell <joe@tidbits.com>

Enter the keyboard shortcut you want to use and click Add; repeat if necessary for multiple accounts. The next time you’re composing a message and realize you want to switch the From account, press the corresponding keyboard shortcut.