451 please retry later message but it doesn't seem to be retrying

[Guest]

[jah]

We use Endian as our firewall and Exchange behind it as the email server. There is a specific domain using greylisting and whenever we send them email it comes back undeliverable with a "451 please retry later" message. The users never get our email, so it seems the server is not retrying like it should.

Any ideas?

Thanks in advance,

Jamie