As you historically Exchange was always using the Primary Email Address when sending email, even when the user choose to edit the From field to use one of the aliases associated with the mailbox.
The only workarounds where either to use SMTP Auth connection – which most of the time is disabled for end-user mailboxes – or shared mailbox – which introduce complexity for both administrators and users as this implies additional mailboxes.
Well, good news a first step to allow end-user sending emails using alias is now available.
Once enabled in your tenant, the feature will be available only from Outlook Web Access (aka Outlook on the Web) and Outlook mobile; support for Outlook on Windows / Mac will come later.
To enable the feature, Exchange Online administrator must connect with the Exchange Online PowerShell and run the below command:
Set-OrganizationConfig –SendFromAliasEnabled $true
Then using Outlook Web Access, end-users can define the list of alias being available for sending email by accessing the Outlook Settings using the Gear button.
The option Addresses to send from is available under the Mail\Compose and reply section
After selecting the aliases to be available, end-user can then select one of them using the From field when composing a new email
Do not use the Other email address to manually enter an alias not selected from the Settings
This will be the same operation when using Outlook Mobile; you will still need to first use OWA to define the list of aliases to use.