I wrote a moment ago:
> Dotlocking will create a file /var/spool/mail/vdanen.lock.
> In order to do this, mutt_dotlock needs write access to
> that directory, which is normally enabled by making
> mutt_dotlock setgid mail.
To illustrate things, here's an example from my system:
% ls -ld /var/spool/mail /var/spool/mail/roessler /usr/local/bin/mutt_dotlock
-rwxr-sr-x 1 root mail 24231 Jun 20 10:32 /usr/local/bin/mutt_dotlock
drwxrwsr-x 2 root mail 1024 Jun 8 16:56 /var/spool/mail
-rw-rw---- 1 roessler mail 0 Apr 18 16:19 /var/spool/mail/roessler
(The spool file is actually unused since my mail gets
delivered to a maildir folder in $HOME.)
