Don't create multipart/alternative messages with empty text/plain part (#5283)

CHANGELOG

@@ -23,6 +23,7 @@ CHANGELOG Roundcube Webmail
 - Fix subscription checkbox state on error in folder subscribe/unsubscribe action (#5243)
 - Fix bug where microsecond format in logged date didn't work in some cases
 - Fix conflict in new_user_dialog and password_force_new_user settings (#5275)
+- Don't create multipart/alternative messages with empty text/plain part (#5283)
 
 RELEASE 1.2.0
 -------------

program/steps/mail/sendmail.inc

@@ -388,19 +388,19 @@ if ($isHtml) {
     $plainTextPart = rcube_mime::wordwrap($plainTextPart, $LINE_LENGTH, "\r\n", false, $message_charset);
     $plainTextPart = wordwrap($plainTextPart, 998, "\r\n", true);
 
-    // completely blank text part confuses some mail clients
-    if ($plainTextPart == '') {
-        $plainTextPart = "\r\n";
-    }
-
-    // make sure all line endings are CRLF (#1486712)
-    $plainTextPart = preg_replace('/\r?\n/', "\r\n", $plainTextPart);
+    // There's no sense to use multipart/alternative if the text/plain
+    // part would be blank. Completely blank text/plain part may confuse
+    // some mail clients (#5283)
+    if (strlen(trim($plainTextPart)) > 0) {
+        // make sure all line endings are CRLF (#1486712)
+        $plainTextPart = preg_replace('/\r?\n/', "\r\n", $plainTextPart);
 
-    $plugin = $RCMAIL->plugins->exec_hook('message_outgoing_body',
-        array('body' => $plainTextPart, 'type' => 'alternative', 'message' => $MAIL_MIME));
+        $plugin = $RCMAIL->plugins->exec_hook('message_outgoing_body',
+            array('body' => $plainTextPart, 'type' => 'alternative', 'message' => $MAIL_MIME));
 
-    // add a plain text version of the e-mail as an alternative part.
-    $MAIL_MIME->setTXTBody($plugin['body']);
+        // add a plain text version of the e-mail as an alternative part.
+        $MAIL_MIME->setTXTBody($plugin['body']);
+    }
 
     // Extract image Data URIs into message attachments (#1488502)
     rcmail_extract_inline_images($MAIL_MIME, $from);