lib/PublicInbox/POP3.pm | 3 ++-

diff --git a/lib/PublicInbox/POP3.pm b/lib/PublicInbox/POP3.pm
index 8612356329fde8ac4584221b071efde1bd63dbeb..51c2b71a560fd10e041ad8bb949e2b8c3ab6486e 100644
--- a/lib/PublicInbox/POP3.pm
+++ b/lib/PublicInbox/POP3.pm
@@ -176,7 +176,7 @@ 	if ($self->{pop3d}->lock_mailbox($self)) {
 		$self->{uid_max} = $self->{ibx}->over(1)->max;
 		\"+OK logged in\r\n";
 	} else {
-		\"-ERR unable to lock maildrop\r\n";
+		\"-ERR [IN-USE] unable to lock maildrop\r\n";
 	}
 }
 
@@ -350,6 +350,7 @@ USER\r
 PIPELINING\r
 UIDL\r
 EXPIRE 0\r
+RESP-CODES\r
 .\r
 EOM
 }