Relaying error when sending a receipt

[Brian Denton]

Hello,

Whenever I try to send a "read receipt", I get the following error:

"The message could not be sent because one of the recipients was rejected by
the server. Server Response: '550 relaying mail to <my_domain> is not
allowed'. (Account: '<My Account>', SMTP Server: <our_proxy_server>, Error
Number: )x800ccc79).

Is it because I'm behind a firewall/Proxy server? If so, is there anything
I can do, either in Outlook or on the Proxy Server, to correct this?

Thanks

Brian