unable to lock and fopen /var/spool/mail/jeff

I've been trying to set up nmh (exmh) on my Slackware Linux
(2.0.33). I've ran into a small problem when trying to 'inc'
The error I get is 'unable to lock and fopen
/var/spool/mail/jeff'

Do I need to change the permissions of this file. It now
reads:

-rw-rw----     jeff     mail     4932 Mar29 09:12 jeff

One other question, I am trying to learn about using all the
different programs (fetchmail, procmail, sendmail, nmh
(exmh)). I don't quite understand something. Is procmail and
nmh the same type of programs?
I have setup procmail (via sendmail) as my local mail
delivery agent.


Thanks.

--
Jeff Bell

Research is what I'm doing when I don't know what I'm doing.
                        -- Wernher von Braun --