OWA Adds Full Support for Send from Proxy Address (Mailbox Aliases)

Client-Side Completion for a Server Feature Delivered in April 2021

Exchange Online delivered support for sending email from proxy addresses (mailbox aliases) in April 2021. At least, the server-side part of the equation was happy to process messages sent using proxy addresses. It can take a little time for clients to catch up. A proxy address is an SMTP address assigned to a mailbox. Usually, people use the primary SMTP address for all outgoing messages (and the primary address is often the same as their Microsoft 365 sign-in address, or user principal name, so it’s easily remembered). Secondary proxy addresses are used in situations like:

  • Mergers and acquisitions: Email addresses like the ones used at an acquired company.
  • Life events: Email addresses with previous versions of someone’s name (like a married surname).
  • Organizational addresses: Email addresses with functional titles, such as a departmental manager.

Exchange Online can deliver email addressed to any proxy address to the mailbox.

Supposed to Happen in May

MC252942 (published April 27) announced that OWA would support sending from proxy addresses in early May. Well, that didn’t happen, which was surprising because OWA is usually at the forefront of delivering new features for Exchange Online. Being able to develop and publish new functionality more quickly than Outlook desktop can is one of the reasons why OWA is core to Microsoft’s One Outlook initiative.

Microsoft 365 roadmap item 59437 describes the feature as “rolling out” starting in August. I haven’t seen the new setting appear until this week, so it’s possible that its deployment is still in progress.

OWA Option to Select Proxies

It was possible to send email previously with OWA using a proxy address by typing the proxy address in the From field in the new message screen. However, people often don’t know what proxy addresses are available to them, so while OWA could send messages from proxies, it wasn’t as easy as it could be. This has now been addressed with the arrival a new setting in OWA options to allow users to choose the set of proxy addresses they would like to use (Figure 1).

Selecting proxy addresses to send from in OWA options
Figure 1: Selecting proxy addresses to send from in OWA options

The address at the top of the list is the primary SMTP address for the mailbox. In my case, it appeared twice (a bug). A user can’t unselect their primary SMTP address as it remains the default for sending. If they want to use a different primary address, they’ll have to ask an administrator to update their account. It’s good practice to make the previous primary address a secondary proxy to ensure that Exchange Online can deliver responses or new messages sent to that address.

It’s wise to check the Always show From option in OWA settings as this will make it easier to select one of the chosen proxies.

Sending From a Proxy

Once you’ve selected the set of proxy addresses that you’d like to use (and made sure that the From field is exposed), it’s easy to select a proxy to use because OWA shows the available set in a drop-down list (exposed by clicking the From button) in the compose message screen. The list (Figure 2) includes:

  • The primary SMTP address for the mailbox (the default).
  • The proxy addresses selected in OWA options.
  • Other addresses previously used to send email. Typically, these are addresses for other user and shared mailboxes where the user has Send As or Send on Behalf of permission.
  • The option to add another email address (for instance, for a mailbox you’ve just been given the permission to send from).

 Selecting a proxy address to send a message from
Figure 2: Selecting a proxy address to send a message from

Select the proxy to use and compose the message as normal. When Exchange Online sends the message, it populates the From and Return-Path values in the message header (Figure 3).

The Return-Path value in the message header is set to the proxy address
Figure 3: The Return-Path value in the message header is set to the proxy address

Small Changes Make a Difference

There’s nothing earthshattering in this update. The Exchange transport system has always been able to deliver email addressed to any valid proxy address assigned to a mailbox. The heavy lifting occurred in April when Exchange Server updated transport to allow it to accept messages sent from proxy addresses. It’s nice that OWA is making it easier for people to use the feature.


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