Documentation/standards.perl | 1 +
 lib/PublicInbox/IMAP.pm      | 7 +++++--

diff --git a/Documentation/standards.perl b/Documentation/standards.perl
index 8fc852c722bae5e48d6bcd3686ee29f7ea442662..b4bda8a9c06801df7be144e5606c66b7ce8017b6 100755
--- a/Documentation/standards.perl
+++ b/Documentation/standards.perl
@@ -44,6 +44,7 @@ 	2822 => 'Internet message format (2001)',
 	5322 => 'Internet message format (2008)',
 	3501 => 'IMAP4rev1',
 	2177 => 'IMAP IDLE',
+	2683 => 'IMAP4 Implementation Recommendations',
 	# 5032 = 'WITHIN search extension for IMAP',
 	4978 => 'IMAP COMPRESS Extension',
 	# 5182 = 'IMAP Extension for Referencing the Last SEARCH Result',
diff --git a/lib/PublicInbox/IMAP.pm b/lib/PublicInbox/IMAP.pm
index b3c449b0712778b571ab2cbbb0d5e24b72e53c65..2e50415de57367b4f6b996c1f372ff21f1042ceb 100644
--- a/lib/PublicInbox/IMAP.pm
+++ b/lib/PublicInbox/IMAP.pm
@@ -36,7 +36,7 @@ 	$Address = $mod and last;
 }
 die "neither Email::Address::XS nor Mail::Address loaded: $@" if !$Address;
 
-sub LINE_MAX () { 512 } # does RFC 3501 have a limit like RFC 977?
+sub LINE_MAX () { 8000 } # RFC 2683 3.2.1.5
 
 # changing this will cause grief for clients which cache
 sub UID_BLOCK () { 50_000 }
@@ -1170,7 +1170,10 @@
 	my $rbuf = $self->{rbuf} // \(my $x = '');
 	my $line = index($$rbuf, "\n");
 	while ($line < 0) {
-		return $self->close if length($$rbuf) >= LINE_MAX;
+		if (length($$rbuf) >= LINE_MAX) {
+			$self->write(\"\* BAD request too long\r\n");
+			return $self->close;
+		}
 		$self->do_read($rbuf, LINE_MAX, length($$rbuf)) or return;
 		$line = index($$rbuf, "\n");
 	}