Yep, as it says, they've cocked up by putting in an IP address into the MX record.
What they should have done is put a FQDN into the MX record which pointed to a valid A-record, rather than an IP address.
i.e. your DNS now looks like:
aberystwyth.gov.uk MX preference = 5, mail exchanger = 213.171.216.114
and that "mail exchanger" bit at the end should NEVER be an IP address, it should be the Fully Qualified Domain Name (FQDN) of a mailserver.
Hope that helps.
Mike.