4.7.7 Delay Email

[MSR63]

I have some external folks (not all) telling us that are getting the following error:

Could not deliver the message in the time limit specified.
> Please retry or contact your administrator.
> < prserv.net
> #4.4.7Unabletocontacthostfor1days,-original_message_forwarded
> smtp;
> Persistent Transient Failure: Delivery time expired>

Any suggestions - this exchange server has been up and running for over a year and there has been no recent changes.

Thanks