When posting this type of question, you'll generally get a better response if you take a shot and post the code that's not working for you. That said, you might want something like this:

map s/\@ulster\.ac\.uk//, @your_array;
[download]

or

map s/\@ulster\.ac\.uk//g, @your_array;
[download]

if there is a possibility that an array element will hold more than one email address.

Take some time and look up 'map' and s///. There is MTOWTDI, and there are more basic solutions to this problem.