This article explains how to permanently or temporarily change the "From" email address in Outlook on the web and Outlook for Microsoft 365.

If you often change the From line in Outlook.com one email at a time, consider setting a default From address to save time. This way, all new emails will automatically be sent from the associated account you select, and the address will appear in the From field. To specify the email address that should be used by default in the From field of messages you compose using Outlook.com:

Select Settings (the gear icon in the top navigation bar).

Select View all Outlook settings.