unknown user

Hi there, On our PMG Cluster, we have 1 node that rejects all mail for one of our users. Here is a NDR: node1 said reject: RCPT from smtp2.e.amses.net[213.161.89.103]: 550 5.1.1 <nigel@example.co.uk>: Recipient address rejected: undeliverable address: unknown user: "nigel"...