Send from an Alias – Shared Mailbox

Say you have your primary email [email protected] and an alias [email protected]

If you want to send as [email protected] you need to "Configure Exchange Online to Send from Aliases" (see below)

Caveats

With a Shared mailbox sending from an alias only works in Outlook Web Access

If you normal Exchange Online Mailbox has aliases you can send from your Outlook Desktop Client and OWA

Powershell

Connect-ExchangeOnline
Set-organizationconfig -SendFromAliasEnabled $true

Or using the ECP

Choose Settings on the left pane in the Exchange Admin Panel

Select Mail Flow

Check "Turn on sending from aliases"

Save