Ok - I think I understand!!
They have replied to me and said-
"Thanks for contacting us with your support enquiry.
Because you just point the mx-record of aberystwyth.gov.uk to an IP address, the emails are bouncing back to you. Other mail providers will consider this as invalid mx-record. To rectify the issue, you need to create a glue domain ( e.g mail.aberystwyth.gov.uk ) in your current provider and have it pointed to 213.171.216.114. Once done, you need to set the glue domain as the mx-record of aberystwyth.gov.uk. This should resolve the issue.
Normally, change in DNS will take 24-48 hours to take effect."
Is this right?
What I dont understand is that some mail gets in and out some not! I dont understand why its different!
Thanks for the help so far.