clarify what the other alternatives section means

policy/tpa-rfc-15-email-services.md

@@ -625,6 +625,11 @@ server.
 
 # Other alternatives
 
+Those are other alternatives that were considered as part of drafting
+this proposal. None of those options is considered truly viable from a
+technical perspective, except possibly external hosting, which remains
+to be investigated and discussed further.
+
 ## No mailboxes
 
 An earlier draft of this proposal considered changing the