Hi,
Not sure if this is new, I just discovered that if you change the outgoing mail server to iCloud, then save the setting, you'll be able to add send as Alias in Mail. (See the screenshots).
At least it worked with Exchange Online accounts, with Send As permission enabled.(Distribution Group, or Shared Mailbox)
After you added the alias, make sure you change the outgoing mail server back to "Exchange".
Not sure if this is new, I just discovered that if you change the outgoing mail server to iCloud, then save the setting, you'll be able to add send as Alias in Mail. (See the screenshots).
At least it worked with Exchange Online accounts, with Send As permission enabled.(Distribution Group, or Shared Mailbox)
After you added the alias, make sure you change the outgoing mail server back to "Exchange".