
In my tests, accounts with non-Microsoft addresses added as aliases and the new custom domains are affected. I will not respond to messages sent to those addresses (and probably won't even see them.) Note: those addresses are only used for testing. (Exchange users have wanted the ability to send using the secondary addresses assigned to their mailboxes for years but it's only available in accounts at this time. My best guess on the cause of this bug is that it is related to a new Outlook 2016 feature for accounts that lists all aliases for your account on the From selector, allowing you to use any of select any of the aliases to send from. Yes, it's an extra step that should not be necessary.


Test it, and if it sends from the wrong account, select the correct account using the From selector. At this time, the correct From account is actually used, even though it displays the wrong account.

New messages use the correct email account but replies or forwards always show as From the account.
