[Exim] Error Messages

Hi,

I have exim on a server which is acting as an outgoing mail relay for
customers. Recently I have been informed of someone getting error messages
when sending messages, and I am not able to understand why exim is behaving
in that way. It keeps cucking up the following error:

451 Cannot Check <email address here> at this time please try later

From what I can see, the documentation says that a 451 should be expected
on incoming messages only, and as this is not a local domain I cannot
understand why we should see this error.

Can anyone shed any light on the subject?

Many Thanks

Andrew