specialized subdirectory

[j.travis]

I would like to create a specialized subdirectory whenever useradd is invoked.  More specifically:

for a user:          user1
with a home directory:     /home/share

I would like to automatically create:

/home/share/.user1-mail/      

when I issue:

useradd -d /home/share user1            

I have been looking through the files in /etc/profile /etc/skel /etc/bashrc and whatnot but haven't found the right spot yet.  Any ideas?