This is from my mail logs after requesting emails from the site, maybe it can help?
2006-09-05 11:31:41 H=(www.photopost.com
) [184.108.40.206] F=<email@example.com> rejected RCPT <firstname.lastname@example.org>: Sender verify failed
verify fail for <email@example.com>: response to "RCPT TO:<firstname.lastname@example.org>" from mail-server.com [220.127.116.11] was: 511 sorry, no mailbox here by that name (#5.1.1 - chkuser)
This liscense is rather useless if i cant receive emails to download it...