The check_vpopmail program uses the libvpopmail for external authentication
in ejabberd.  The vpopmail has to be installed in order for the program to 
compile.  Vpopmail is assumed to reside under /mail.

REFERENCES

vpopmail
- http://www.inter7.com/vpopmail/vpopmail.html

ejabberd
- http://ejabberd.jabber.ru