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Microsoft 365 (Outlook on the web | Outlook for Windows/Mac) - Can you send mail using an alternate address
Office 365 will always use your primary address as your "from" and "reply-to" settings when you send messages via Outlook on the web or Outlook for Windows/Mac when configured using Exchange protocol.
If you want to change this so that it uses one of your alternate addresses instead, you will need to modify your primary address. Then all messages sent via Outlook on the web or Outlook desktop client will use this new primary address for you "from" and "reply-to" settings.