Error 550. DNS Issue?

Hey all,

We are going to be upgrading our Domino mail server from 6.5 to 8.5. What we would like to achieve first is to create a test email environment. This will be a internal mail server routing with a test domain name using the same external IP address as our live domain. If you are wondering why we want to do this, 6.5 to 8.5 is a big step and our email server is on it's last legs and badly configured. To minimize downtime while we test, we need a working test environment.

realdomain.co.uk (not the real domain name)
MX records:
relay1.mailwash.co.uk (our mail filtering service)
relay2.mailwash.co.uk

NS records:
ns0.bt.net
ns1.bt.net
ns2.bt.net

I have registered a test domain from 1&1. Lets say its testdomain.co.uk
MX records:
mx00.1and1.co.uk
mx01.1and1.co.uk

NS records:
ns67.1and1.co.uk
ns68.1and1.co.uk

Firewall access rules and NAT policies are in place to route the correct MX records to the correct internal mail servers via SMTP. I can send emails from my test domain to my Hotmail account no problems. If I try and send an email address from my Hotmail to an email address I have configured on my test domain I receive a 550 error.

Little more details on the error:
<Init> 220 mx-b.kundenserver.de (mxbap3) Welcome to Nemesis ESMTP server
<Helo> 250 mx-b.kundenserver.de
<MailFrom> 250 OK
<RcptTo> 550 <user.name@testdomain.co.uk>: invalid address
<Quit> 221 OK

I believe this is a DNS issue, unfortunately I am not an expert in DNS. How do I point the MX records point to our external IP address? Do I need to do something with the NS records for our testdomain.co.uk? I'm not sure what to do next. Any help/pointers will be much appreciated thank you.

Ok so problem sorted, it was a DNS problem.

I spoke to my ISP and got transfered to a guy who said he likes to change DNS records for fun!

He gave me a new MX address for our 1&1 test domain, updated 1&1, updated NAT rule, flushed the dns on dns and dev mail server and everything worked. I assume the MX record he gave me must have some sort of forward set to forward to our external IP address.

If anyone has any sort of DNS changes to make then the ISP is the best place to go.