Index: program/steps/mail/func.inc
===================================================================
--- program/steps/mail/func.inc	(revision 64233d21890da61b4052f4ffd6bc34e4a83f5bc6)
+++ program/steps/mail/func.inc	(revision 1d8cbca2720171a2f0fe07ec7f34a5c637fae38e)
@@ -1545,5 +1545,5 @@
       'To'   => $message->headers->mdn_to,
       'Subject' => rcube_label('receiptread') . ': ' . $message->subject,
-      'Message-ID' => sprintf('<%s@%s>', md5(uniqid('rcmail'.mt_rand(),true)), $RCMAIL->config->mail_domain($_SESSION['imap_host'])),
+      'Message-ID' => rcmail_gen_message_id(),
       'X-Sender' => $identity['email'],
       'References' => trim($message->headers->references . ' ' . $message->headers->messageID),
@@ -1584,4 +1584,27 @@
 
   return false;
+}
+
+// Returns unique Message-ID
+function rcmail_gen_message_id()
+{
+  global $RCMAIL;
+
+  $local_part  = md5(uniqid('rcmail'.mt_rand(),true));
+  $domain_part = $RCMAIL->user->get_username('domain');
+
+  // Try to find FQDN, some spamfilters doesn't like 'localhost' (#1486924)
+  if (!preg_match('/\.[a-z]+$/i', $domain_part)) {
+    if (($host = preg_replace('/:[0-9]+$/', '', $_SERVER['HTTP_HOST']))
+      && preg_match('/\.[a-z]+$/i', $host)) {
+        $domain_part = $host;
+    }
+    else if (($host = preg_replace('/:[0-9]+$/', '', $_SERVER['SERVER_NAME']))
+      && preg_match('/\.[a-z]+$/i', $host)) {
+        $domain_part = $host;
+    }
+  }
+
+  return sprintf('<%s@%s>', $local_part, $domain_part);
 }
 
Index: program/steps/mail/sendmail.inc
===================================================================
--- program/steps/mail/sendmail.inc	(revision e6ce0062f2331b8756cc91944ceaea8d7cbffd18)
+++ program/steps/mail/sendmail.inc	(revision 1d8cbca2720171a2f0fe07ec7f34a5c637fae38e)
@@ -206,5 +206,5 @@
   $olddraftmessageid = get_input_value('_draft_saveid', RCUBE_INPUT_POST);
 
-$message_id = sprintf('<%s@%s>', md5(uniqid('rcmail'.mt_rand(),true)), $RCMAIL->config->mail_domain($_SESSION['imap_host']));
+$message_id = rcmail_gen_message_id();
 
 // set default charset