How would I do this ...

well I'd forget about listboxes and their related  functions.
Just do it from files.


read in a line, chop it about @ and save it in a 2d arraylist (or array).

then you can use super fast code to loop and compere.

I don't really understand your criteria for what is or isn't allowed .... if the domain isn't important then just chop around @ and save the first half.

The just loop through one array checking if it exists in the other.

Or there are bubble sorts and so forth available - or there are array sorts available withinn .NET that can get rid of duplicate and so forth.

Should be pretty easy. Enjoy the exploration.

To convert a form to a control just change it to a UserControl and get rid of main, and autoscalebasesize and if you have any menus change them to context menus.
Set project to compile as as dll and voila ! Bob your uncle ! (British expresssion, soz)