Example code please?

i suspect you just used the mailbox name as
mbox parameter instead of the full qualified
form "{servername}path_to_mailbod}"

see example below:

<?php
  $stream = imap_open("{your.imap.host}INBOX.Drafts","username", "password");

  $check = imap_check($stream);
  print "Msg Count before append: ". $check->Nmsgs."\n";

  if(! imap_append($stream,"{your.imap.host}INBOX.Drafts"
      ,"From: me@my.host\r\n"
      ."To: you@your.host\r\n"
      ."Subject: test\r\n"
      ."\r\n"
      ."test\r\n"
      ))
    echo "Append faild: ".imap_last_error()."\n";

  $check = imap_check($stream);
  print "Msg Count after append : ". $check->Nmsgs."\n";

  imap_close($stream);
?>

no feedback and not reproduceable
in latest php 3 and 4