Allow messages with addresses in BCC only.

src/compose.php

@@ -993,9 +993,9 @@ function checkInput ($show) {
      * using $show=false, and then when i'm ready to display the error
      * message, show=true
      */
-    global $body, $send_to, $subject, $color;
+    global $body, $send_to, $send_to_bcc, $subject, $color;
 
-    if ($send_to == '') {
+    if ($send_to == '' && $send_to_bcc == '') {
         if ($show) {
             plain_error_message(_("You have not filled in the \"To:\" field."), $color);
         }
@@ -1103,4 +1103,4 @@ function getReplyCitation($orig_from)
     return ($start . $orig_from . $end . "\n");
 }
 
-?>
+?>