By default, with Microsoft Exchange, any email sent is sent with the primary address of the mailbox used (or the shared mailbox used).
It is now possible with MS365 to allow users to send emails from secondary addresses.
To do this, you must activate a new option available in Exchange Online.
2. Allow sending email with an SMTP alias
The new “SendFromAliasEnabled” parameter defaults to $false value.
You must set this parameter to $true with the following command:
Set-OrganizationConfig -SendFromAliasEnabled $true
Your users can then choose the email address for sending emails and Exchange Online will not change the address to the user’s primary address when sending.
To do this, you must always use the “From” button in Outlook,
or the “From” button in Webmail: