The early release of a useful new feature is always appreciated, even if I suspect that the feature is only released on the Exchange Online side at present with some client-side work still to come to make the task easier to perform. This is Microsoft 365 roadmap item 59437, which the roadmap says is due for release in June 2021. This week, Microsoft rolled out an update to allow clients to send messages using any SMTP proxy address assigned to a user’s mailbox. Exchange Online can accept and deliver email addressed to any proxy address for a mailbox, but up to now users have been unable to use their non-primary SMTP addresses when sending messages. One of the addresses, called the primary SMTP address, is stamped on outbound messages. Exchange Online mailboxes can have multiple SMTP proxy addresses (also known as aliases).
