diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index 5be6173f34..b9c2f30a54 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -600,7 +600,7 @@ def recipient_for_emails(emails, not_forged_mirror_message,
     if len(realm_domains) == 2:
         # I'm assuming that cross-realm PMs with the "admin realm" are rare, and therefore can be slower
         admin_realm = Realm.objects.get(domain=settings.ADMIN_DOMAIN)
-        admin_realm_admin_emails = [u.email for u in admin_realm.get_admin_users()]
+        admin_realm_admin_emails = {u.email for u in admin_realm.get_admin_users()}
         # We allow settings.CROSS_REALM_BOT_EMAILS for the hardcoded emails for the feedback and notification bots
         if not (normalized_emails & admin_realm_admin_emails or normalized_emails & settings.CROSS_REALM_BOT_EMAILS):
             raise ValidationError("You can't send private messages outside of your organization.")