conditioning on part of the email address

Let's say, I am running a little family project and according to the 
surname of the email address I want to change to a different sub
directory.

1) Father(_dot_)Family(_at_)sameHost(_dot_)org
2)Mother(_dot_)Family(_at_)sameHost(_dot_)org

So how do I cut out the "mother | father" and then change the
subdirectory?

O!