I believe the following patch (against the current main branch) could solve this issue by skipping validations when email_in_allow_strangers is set for the category, in the same way it is skipped for staged users. Does that sound sensible?
diff --git a/lib/email/receiver.rb b/lib/email/receiver.rb
index 7c76c44d61..dd3bc3cfb0 100644
--- a/lib/email/receiver.rb
+++ b/lib/email/receiver.rb
@@ -762,7 +762,7 @@ module Email
elided: elided,
title: subject,
category: destination.id,
- skip_validations: user.staged?)
+ skip_validations: (user.staged? || destination.email_in_allow_strangers))
elsif destination.is_a?(PostReplyKey)
# We don't stage new users for emails to reply addresses, exit if user is nil