The issue was one between the semi-competent 123-reg sysadmin and the semi-competent spam stopers at the university. 123-reg mail relays keep getting on spam blacklists because they sometimes relay spam. spam stoppers block blacklisted IP addresses.

123-reg people say "we relay mail, some of it is spam -- we can't help being on blacklists sometimes"

spam people say "they are on blacklists -- if you give us IP numbers of servers we can whitelist them for you."

It took me two weeks to persuade 123-reg to provide me the IP nos of their servers and I had to ask them a few times to get themselves removed from the spam blacklist.

A few months after that they changed to new servers which ended up on blacklists.