Delivered Receipts

[Julian]

When I receive a message requesting that a read receipt be
sent back to the original sender, I say "Yes" but the read
receipt stays in the Outbox and cannot be sent. I get an
error message saying"The message could not be sent because
one of the recipients was rejected by the server. SErver
response: '554 <sender's e-mail>: Recipient address
rejected: Relay access denied'

Does anyone know why this happening ?

Thanks