Modify titles of subscription emails

Hello,
We have a request from a number of subscribers to the forum. They would like to add a prefix [IUG] to the front of every email Subject that is sent from the site. This way they can be filtered and organised correctly in their mail software as some are reciving a lot of emails mixed with their other email.

Which file would I need to modify to add a prefix? perhaps this can also be a feature request for future releases?

Thanks