backup MX problem - 550 error

[Snowman30]

Ive got a weird issue that has just popped up

ive got a client runnign a remote mailserver and he has his MX 0 record pointing to his server and it works fine

However we set him up a backup MX record on his account so that mail would be sent to our server and queued if his mailservice was down

this worked well for ages and now we are getting the error:

550-permitted to relay through this server. Perhaps you have not logged into
550-the pop/imap server in the last 30 minutes or do not have SMTP
550 Authentication turned on in your email client.

his domain is setup in the /etc/remotedomains file and not in /etc/localdomains obviously but his mail shoul still be able to be queued on our server using his MX 10 record but it wont

anyone have any ideas as to whats causing this? and how to get around it?

[Snowman30]

never mind founf the issue for some reason the domain dropped form the /etc/secondarymx file