This is accomplished by no longer replacing aliases during the sending or delivery of messages in Exchange Online. To eliminate the need for these suboptimal solutions, we have introduced new capabilities in Exchange Online that enable Outlook clients to use and preserve aliases and to display the original alias used to send the message. Shared mailboxes could be used, but that meant another identity and another Inbox, as well. They could use the SMTP AUTH client submission protocol, which didn’t rewrite the From addresses. In the past, those wanting to send from aliases had a few options. We are excited to announce that we’ve reached the first milestone of this journey. To address these challenges, we started a journey toward making aliases first-class addresses in Exchange Online. Today, business operations are much more challenging and complex, and activities such as mergers and acquisitions, rebranding, and other such changes have created the need for multiple identities and SMTP domains to be managed by an organization. Even when an email client tries to use an alias for the From address, that value is overwritten with the user’s primary SMTP address when the message is sent. Other SMTP addresses assigned to a user (e.g., proxy addresses, also known as aliases) were mainly intended for receiving messages. Historically, Exchange has used a user’s primary SMTP address as the From address when sending messages.

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