Random Recipient Address Rejected errors

[Jeremey]

Hello,

I have a client who uses Outlook 2k3 with 3 different users mailboxes setup
as additional SMTP/POP3 connections. All of them receive and deliver to the
correct folder, yet randomly she will receive a 554 error stating Recipient
Address Rejected, Access Denied. She will attempt to send it about 6 or 7
times and it will finally go through.

Any thoughts.

Thanks,
Jeremey