Mail Delivery failing on one server only

[coolemail]

Please can someone help. We have two servers both with:
OS Linux 2.6.32.28-1.art.x86_64
Panel version 10.2.0

On one, we are able to send emails to xxx@dmcadmin.co.uk
On the other one, it fails with:

Jun 16 21:45:50 plesk3 qmail: 1308257150.872860 starting delivery 1581: msg 105971977 to remote xxx@dmcadmin.co.uk
Jun 16 21:45:50 plesk3 qmail: 1308257150.915956 delivery 1581: failure: Sorry,_I_couldn't_find_a_mail_exchanger_or_IP_address._(#5.4.4)/

Can someone tell me what setting will allow these emails to go through?

Many thanks in advance.