/usr/bin/mutt_dotlock should be SGID mail to work properly, but it was installed just as "-rwxr-xr-x 1 root root". Without this mutt always opens mailboxes in /var/spool/mail in readonly mode. This is mutt-1.4-r3 on Gentoo linux 1.4-rc2.
Which version of mailbase you are using? If it is mailbase-0.00-r5, it is not a bug. Mutt select lowest possible permissions in time of compilation. Because /var/spool/mail is world-writable, you don't need SGID permissions. Consequence of bug #16749.
I just upgraded to mailbase-0.00-r6 and afterwards had to change the group and SGID of mutt_dotlock. My /var/spool/mail is 775, so it isn't world-writeable (which is probably a good thing) but perhaps the mutt ebuild should make the SGID change by default now.
Oh I just read the other comment and yes re-emerging mutt also fixed the problem.
Bug #16749 is fixed, closing the bug.