Mutt not seeing new mail at all

[Chris G]

I'm running mutt 1.5.18 in ubuntu 8.10 and it's not seeing new mail at
all. That is the status line is not reporting new mail when it arrives
and 'c' doesn't move me to folders with new mail in them.

My muttrc file is one I have transferred (with necessary changes) from
another mutt installation where mutt is working perfectly OK,
detecting new mail etc.

I've never had this problem before with mutt on a variety of platforms
(slackware, fedora, bsd, solaris) and I can't see anything obvious
wrong.

Should mutt work correctly with mbox mailbox files on Ubuntu or has
Ubuntu done something wierd that stops it working?

-- 
Chris Green