A portal that says mail is on now proves it at startup

Mail could be switched on (Email:Enabled=true) with its settings half-filled in, and everything looked fine: the portal started, every page served, the health check was green — and not one message went out. Invitations, notifications and shared documents piled up unsent, with nothing on any screen to say so. The only sign was a single line in a server log.

Now the portal checks that claim when it starts. If mail is switched on but a setting it needs is missing, the portal refuses to start and says exactly which ones — in both the form the documentation uses (Email:TenantId) and the form an operator actually sets (Email__TenantId) — along with every way out: fill them in, switch to a managed identity, or turn mail off.

Turning mail off is still a complete, supported answer, and an installation that never wanted mail is untouched: a blank or absent mail section starts exactly as before. Only an installation that claims mail it cannot send is stopped, and it was never sending anything to begin with.