Have you ever been in a situation where you need extra email addresses to get more free coupons or something? Do you have a gmail address? Gmail has a nifty feature called filters. They can be used to auto-sort your mail. Just add a plus sign and something else after your address name.
But pretty much, most websites will see filtered addresses as completely different. So while the following addresses are the same, they don't look like it to the form you're filling out.
hello@gmail.com
hello+1@gmail.com
hello+asdfjkl@gmail.com
EDIT: Obviously VBulletin is confused by them, since it didn't auto-link them.