I post this in the Apple Support Forums and also sent a bug report to Apple. I am posting here to see if anyone has a fix or if we can nudge Apple to quickly fixing this. I have a total of eleven (11) e-mail addresses, which include my iCloud account. The majority of the addresses are POP3. I have that many addresses because of my business and each e-mail is for a specific web site. In order to sync my e-mails across the multiple computers I have, I created a rule that would move every e-mail I received to the iCloud account. In Lion, even though the e-mail was moved to the iCloud account, when I replied to an e-mail, it would reply from the account to which the e-mail was sent. Thus, if an e-mail was sent to 'firstname.lastname@example.org', it would be moved to the iCloud account (which would sync it among all computers). Replying to it would be sent from 'email@example.com' and use that server and its settings. This has been broken by Mountain Lion and Mail 6.0. Now, an e-mail is received and moved to iCloud. But, when replying, the reply-from address is now the iCloud address -- NOT the e-mail address to which the e-mail was sent. This STILL works in Lion. I have a machine running Lion and have been using it side-by-side with MountainLion. Now, a couple of things I have discovered. While entering e-mail aliases into the e-mail address of the iCloud settings seems to work, it actually causes a couple of problems. First, it causes the iCloud syncing to not work -- the "Mail" box becomes unchecked in System Preferences for iCloud and won't stay checked so long as the "foreign" (non-iCloud) e-mail address is in the account settings for iCloud. Second, the server settings for the outgoing e-mail get broken. It's weird. E-mails don't send at times. I sincerely hope that Apple fixes this. People may suggest that I use IMAP accounts to sync my e-mails. That is not an option. The web host I use only allows a certain number of processes to run. With all my e-mail accounts, checking my e-mail would easily blow me past that number. Thank you for your attention to this issue. All the BEST! T.