Why Microsoft Reannounced the Send from Email Aliases Feature

Groundhog Day for Send from Proxy Addresses

Microsoft’s January 25 announcement of a public preview for the send from email alias feature certainly confused many people, including me, because the feature first appeared last year. I wrote about the topic for Practical365.com on April 22, 2021 and the news was subsequently covered elsewhere.

Microsoft released message center notification MC252942 on April to confirm that OWA would support sending from a proxy address, with roll-out expected in early May. The feature is also covered in Microsoft 365 roadmap item 59437. According to 59437, sending from email aliases reached general availability in August 2021, only it didn’t. Or maybe it did, in a certain way.

It’s odd that Microsoft should rerelease a previously released feature a long time after it first appeared in tenants, but there is a certain logic behind the story. Here’s my take on the situation.

A Send from Email Proxies Primer

In a nutshell, if you want users to be able to send email from proxy addresses (email aliases), you update the Exchange Online organization configuration:

Set-OrganizationConfig -SendFromAliasEnabled $True

The setting applies to the complete tenant. There’s no way to restrict the ability to send email from aliases to certain mailboxes.

An Exchange Online mailbox can have multiple proxy addresses. Exchange could always deliver email to a mailbox based on any of its proxy addresses. After updating the Exchange Online configuration, users can send email using any of the proxy addresses assigned to their mailbox. The exact mechanism differs from client to client. For instance, in OWA, you must select the proxy addresses that you want to use in OWA settings (Figure 1) to populate a list of available addresses for the From field in the message compose screen.

Selecting email proxy addresses to use with OWA with the send from email alias feature
Figure 1: Selecting email proxy addresses to use with OWA

Sending from email proxies is rated as a preview feature, so changes might happen before Microsoft regards send from email alias as a generally available feature. This is a cloud-only feature that Microsoft doesn’t intend bringing to Exchange Server.

Client Support

The original announcement covered OWA and no other client. This is understandable because client GUI must change to allow users to select a proxy address before sending a message. However, OWA didn’t get full support for send email from aliases until October 2021. Work was going on to upgrade Outlook desktop too, as evident in the beta release notes for Outlook for Windows issued on July 30, which cover a bug fix for tenants where the SendFromAliasEnabled configuration setting is True.

Microsoft’s new text says that OWA and Outlook for iOS and Android support send email from aliases, with plans to support Outlook desktop users by Q2 2022. The obvious conclusion is that it’s taken Microsoft longer than anticipated to update all the Outlook clients. Pulling together feature updates across multiple clients underlines the value of the One Outlook project, which is intended to allow much greater code sharing across Outlook clients, like the way the Room Finder works.

However, I think the delay is more likely due at the server level rather than clients. Exchange has processed email for a very long time; the origins of the code base for the SMTP-based transport service came from Exchange 2000, and although the Exchange Online code base is dramatically different because of new capabilities like support for DANE and DNSSEC and the introduction of a legacy SMTP endpoint, it’s always possible that assumptions existed in code that messages used only the primary SMTP address.

Engineers needed to find every path messages could take through the code to assess if all scenarios support sending from email proxies. If not, they needed to apply a fix. The complexity of the Exchange Online transport service is illustrated by the set of known issues described in Microsoft’s post. I assume that Microsoft will address some if not all these issues when send from email aliases reaches general availability.

My theory is that experience of using the original implementation unearthed several knotty bugs. It has taken Microsoft time to upgrade code in both servers and clients to reach the point where they’re confident that everything works as it should.

All’s Well That Ends Well

I don’t think the Microsoft announcement communicated the situation as clearly as they could have. Acknowledging the previous release would have clarified the matter. However, the fact remains that send from email aliases is a very useful and welcome feature to have, even in its preview state.


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