[exim] Subtracting text files [OT]

There's probably an easy way to do this.

I have 2 files. Both are sorted and unique. The first file has a list.
The second file has a list of things that should not be in the first
file. I want to generate a third that that has everything in the first
file except for what is listed in the second file.

How do I do that?

Thanks in advance