Historically, Exchange has used a user’s primary SMTP address as the From address when sending messages. Other SMTP addresses assigned to a user (e.g., proxy addresses, also known as aliases) were ma...
Updated Oct 21, 2024
Version 11.0
Blog Post
Outlook for Mac just decided that you don't need to send from Aliases, it just uses whatever you signed in with to send.
So when a customer was just signed in with mailto:email address removed for privacy reasons instead of their proper UPN mailto:email address removed for privacy reasons - Outlook for Mac INSISTED on sending out of the OnMicrosoft email address - even though the primary SMTP and Reply-To address were both set to the verified Custom Domain.
It worked fine in OWA, but the updated Outlook for Mac refused to send under the correct email until I removed the OnMicrosoft version, closed all Outlook Windows, and re-added it with the correct UPN.
This was all leftover from whenever the migration first came in, but it was never a problem until the recent Outlook for Mac updates