Add hint for generic subject

app/dashboard/templates/dashboard/mailbox_detail.html

@@ -153,6 +153,12 @@
                 this option will allow a further protection of your email content.
               </div>
             </div>
+          <div class="alert alert-info">
+            As the email is encrypted, a subject like "Email for you"
+            will probably be rejected by your mailbox since it sounds like a spam. <br>
+
+            Something like "Encrypted Email" would work much better :).
+          </div>
 
             <div class="form-group">
               <label class="form-label">Generic Subject</label>