You may be on a block list. In common with many Universities, ISS subscribes to some block lists that block mail from known sources of spam. Occasionally, Lancaster University or individuals in the University end up on these international block lists.
How block lists work
Companies running block lists add individuals or institutions to their lists when they receive a lot of reports that those individuals or institutions have been sending out spam.
Getting on a block list accidentally
An individual (or institution) can accidentally end up on a block list when one or more of their computers are compromised by a Trojan controlled by a hacker that sends out spam without the individual being aware of it.
Report block list incidents
Report block list incidents to ISS immediately, with details of who is sending the blocked messages and what they contain (where possible). ISS will look into the issue and request that the owner of the block list unblocks the affected user(s).
Ask the sender of the blocked messages to send them to a private email account (e.g. a private Gmail account).