Sometimes you just need to let a user send from their alias address. In the past, this might have meant making a mailbox to send from and forward incoming emails to the main mailbox, but Microsoft has recently released (Jan 2022) a preview feature for Exchange Online that changes this.

Feature Announcement

Sending From Email Aliases – Public Preview – Microsoft Tech Community

How to do it

Use Exchange Online powershell to connect to Exchange Online, then set the organisation config. Note that this feature is still in preview and should be used with caution.

# Connect to Exchange Online
Connect-ExchangeOnline

Set-OrganizationConfig -SendFromAliasEnabled $true

# or to turn it off...
Set-OrganizationConfig -SendFromAliasEnabled $false

What the user sees

• The From field in Outlook for the Web (OWA) includes aliases, this can be modified by the user in OWA Settings>Mail>Compose and reply>Addresses to send from
• Outlook for Windows does not support this! Microsoft says that “Support for aliases is on the way” but no roadmap or date yet.
• In Outlook iOS & Android, your mileage may vary. It’s meant to show up in a from field somewhere but I couldn’t work it out.

Delegation

From my testing, send as delegation does not give the delegate permission to send as a delegated mailbox’s alias.