warn about the unsupported apple mail configuration

howto/submission.md

@@ -70,6 +70,12 @@ password. You should *NOT* get a certificate warning, a real cert
 
 ## Apple Mail configuration
 
+WARNING: this configuration might not work, because Apple Mail
+actually *requires* an IMAP server for proper operation. The
+configuration below *may* work, but you will probably end up sending
+email through the submission server with an incorrect "envelope
+from", and this will necessarily lead to delivery problems.
+
  1. Click On "Account Information"
 
     * User Name: (your LDAP username, e.g. in my case it is `anarcat`, **without** the `@torproject.org` part)