--- a/lib/Test/Nginx.pm
+++ b/lib/Test/Nginx.pm
@@ -208,21 +208,24 @@ sub testdir() {
 
 ###############################################################################
 
-sub log_out {
+sub log_core {
 	return unless $ENV{TEST_NGINX_VERBOSE};
-	my ($msg) = @_;
-	$msg =~ s/^/# >> /gm;
+	my ($prefix, $msg) = @_;
+	($prefix, $msg) = ('', $prefix) unless defined $msg;
+	$prefix .= ' ' if length($prefix) > 0;
+ 
+	$msg =~ s/^/# $prefix/gm;
+	$msg =~ s/([^\x20-\x7e])/sprintf('\\x%02x', ord($1)) . (($1 eq "\n") ? "\n" : '')/gmxe;
 	$msg .= "\n" unless $msg =~ /\n\Z/;
 	print $msg;
 }
 
+sub log_out {
+	log_core('>>', @_);
+}
+
 sub log_in {
-	return unless $ENV{TEST_NGINX_VERBOSE};
-	my ($msg) = @_;
-	$msg =~ s/^/# << /gm;
-	$msg =~ s/([^\x20-\x7e])/sprintf('\\x%02x', ord($1)) . (($1 eq "\n") ? "\n" : '')/gmxe;
-	$msg .= "\n" unless $msg =~ /\n\Z/;
-	print $msg;
+	log_core('<<', @_);
 }
 
 ###############################################################################

--- a/lib/Test/Nginx/SMTP.pm
+++ b/lib/Test/Nginx/SMTP.pm
@@ -88,6 +88,8 @@ sub smtp_test_daemon {
 		print $client "220 fake esmtp server ready" . CRLF;
 
 		while (<$client>) {
+			Test::Nginx::log_core('||', $_);
+
 			if (/^quit/i) {
 				print $client '221 quit ok' . CRLF;
 			} elsif (/^(ehlo|helo)/i) {
@@ -102,6 +104,8 @@ sub smtp_test_daemon {
 				print $client '500 rcpt to error' . CRLF;
 			} elsif (/^rcpt to:/i) {
 				print $client '250 rcpt to ok' . CRLF;
+			} elsif (/^xclient/i) {
+				print $client '220 xclient ok' . CRLF;
 			} else {
 				print $client "500 unknown command" . CRLF;
 			}

new file mode 100644
--- /dev/null
+++ b/smtp-xclient.t
@@ -0,0 +1,141 @@
+#!/usr/bin/perl
+
+# (C) Maxim Dounin
+
+###############################################################################
+
+use warnings;
+use strict;
+
+use Test::More;
+
+use MIME::Base64;
+use Socket qw/ CRLF /;
+
+BEGIN { use FindBin; chdir($FindBin::Bin); }
+
+use lib 'lib';
+use Test::Nginx;
+use Test::Nginx::SMTP;
+
+###############################################################################
+
+select STDERR; $| = 1;
+select STDOUT; $| = 1;
+
+my $t = Test::Nginx->new()->has('mail')->plan(6)
+	->run_daemon(\&Test::Nginx::SMTP::smtp_test_daemon)
+	->write_file_expand('nginx.conf', <<'EOF')->run();
+
+master_process off;
+daemon         off;
+
+events {
+}
+
+mail {
+    proxy_pass_error_message  on;
+    auth_http  http://127.0.0.1:8080/mail/auth;
+    xclient    on;
+
+    server {
+        listen     127.0.0.1:8025;
+        protocol   smtp;
+        smtp_auth  login plain none;
+    }
+}
+
+http {
+    access_log    off;
+
+    client_body_temp_path  %%TESTDIR%%/client_body_temp;
+    fastcgi_temp_path      %%TESTDIR%%/fastcgi_temp;
+    proxy_temp_path        %%TESTDIR%%/proxy_temp;
+
+    server {
+        listen       127.0.0.1:8080;
+        server_name  localhost;
+
+        location = /mail/auth {
+            add_header Auth-Status OK;
+            add_header Auth-Server 127.0.0.1;
+            add_header Auth-Port   8026;
+            add_header Auth-Wait   1;
+            return 204;
+        }
+    }
+}
+
+EOF
+
+###############################################################################
+
+# When XCLIENT's HELO= argument isn't used, the  following combinations may be
+# send to backend with xclient on:
+#
+# xclient
+# xclient, helo
+# xclient, ehlo
+# xclient, from, rcpt
+# xclient, helo, from, rcpt
+# xclient, ehlo, from, rcpt
+#
+# Test them in order.
+
+# xclient
+
+my $s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('AUTH PLAIN ' . encode_base64("\0test\@example.com\0secret", ''));
+$s->authok('xclient');
+
+# xclient, helo
+
+$s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('HELO example.com');
+$s->read();
+$s->send('AUTH PLAIN ' . encode_base64("\0test\@example.com\0secret", ''));
+$s->authok('xclient, helo');
+
+# xclient, ehlo
+
+$s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('EHLO example.com');
+$s->read();
+$s->send('AUTH PLAIN ' . encode_base64("\0test\@example.com\0secret", ''));
+$s->authok('xclient, ehlo');
+
+# xclient, from, rcpt
+
+$s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('MAIL FROM:<test@example.com>');
+$s->read();
+$s->send('RCPT TO:<test@example.com>');
+$s->ok('xclient, from');
+
+# xclient, helo, from, rcpt
+
+$s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('HELO example.com');
+$s->read();
+$s->send('MAIL FROM:<test@example.com>');
+$s->read();
+$s->send('RCPT TO:<test@example.com>');
+$s->ok('xclient, helo, from');
+
+# xclient, ehlo, from, rcpt
+
+$s = Test::Nginx::SMTP->new();
+$s->read();
+$s->send('EHLO example.com');
+$s->read();
+$s->send('MAIL FROM:<test@example.com>');
+$s->read();
+$s->send('RCPT TO:<test@example.com>');
+$s->ok('xclient, ehlo, from');
+
+###############################################################################