Hi Chris - can you provide some context?
Usually, when I think of SMTP relay, I think of two situations:
- Organizations who have mailboxes in Exchange Online already, so they do not need on-premises Exchange for mailboxes but want it for SMTP relay scenarios (devices etc.) In this case, would all users not be already licensed by cloud licenses, and if so - if they have E3/E5 then you are covered for server licenses on premises?
- Organizations who still have mailboxes on premises and SMTP relay is one of things their Exchange servers need to do (but there are other reasons why they keep Exchange on-premises). In that case, indeed, Exchange servers used for SMTP relay would need to be licensed just as servers hosting mailboxes on premises need to be licensed?
In other words - the "SMTP relay Exchange Server" is not something one would install without either mailboxes in Exchange Online or on-premises, right?