SMTP Protocol Returned a Permanent Error 550 Sender verify failed

Hi,

Trying to get the bottom of this error whilst setting up domino as a SMTP Server.

It looks like it is transferring the mail to the SMTP relay at the ISP, however it is being denied.

Any ideas of what to do?

Log excerpt:

23/04/2004 00:30:30 Router: Transferring mail to domain relay.plus.net (host relay.plus.net [212.159.11.46]) via SMTP

23/04/2004 00:30:34 Router: Error transferring message 007DEF58 via SMTP to relay.plus.net 550 Sender verify failed