NDR errors on email, but only with some senders
We have had an issue with email for a while. We have exchange 2010 behind an isa 2006 firewall. This is then on a virgin media big red connection. Also connected into this connection is our frog vle.
Some of our senders can send email to us with no problems. But we have some senders who can't and they get a ndr return of 554 5.7.1 relay not allowed. The thing is this ndr is not being returned by our exchange server, it is being returned by the ip address from our frog vle. All our mx records on the Internet are correct and external pings, nslookups and trace routes back correct.
We use the frog vle for our website which is using the same domain as our exchange email but not using the email on the frog vle with the same domain.
Has anyone any suggestions or ideas of what this may be? I have paid for engineering time for this issue and it has still not been solved.
Any help would be fantastic.