One of the longest running user requests is finally a reality. Users can finally send emails as one of their alias SMTP addresses. At least they can in Exchange Online in Microsoft 365. We'll need to wait and see if this comes to on-premises Exchange Servers. I can't imagine why it wouldn't, but it will probably only come to Exchange 2019 in a future CU or Exchange Server vNext.
UserVoice may no longer exist, but ancient stone tablets have been unearthed that show this has been requested by users for a long, long time.
Exchange Admins can assign alternate email addresses, or aliases, to user mailboxes. Users can receive emails for any of their aliases, but up till now, emails and replies can only be sent using their primary SMTP email address.
With the new behavior, emails sent using one of their aliases show the From address and the Reply-To address as the alias SMTP address that's being used. And there was much rejoicing.
You enable Send From Alias using Exchange Online PowerShell. Simply run the following cmdlet:
Set-OrganizationConfig -SendFromAliasEnabled $true
Once set, users can send emails using one of their configured alias addresses in Outlook or OWA.
To do this in Outlook, the user must first show the From field for new emails using the Options menu. Then they can pick an alias address they've previously used or click Other Email Address and type in the one they want to use.
To do this from Outlook on the web, create a new message, click the (...) ellipses, and click Show From. Then type the email alias you want to use for the new email.
Emails will be delivered to recipients showing the user's full name and the From email address:
If we examine the SMTP headers we see that the From address and the Return-Path values are using the specified alias.
In my testing I found that when the user receives emails or replies to the alias, Outlook always replies from the primary email address. The user will need to manually change the From address if they want to "continue the lie". Hopefully, this will be fixed in the future.