Ok. My /etc/exim/exim.conf has:
local_delivery:
driver = appendfile
group = mail
mode = 0660
mode_fail_narrower = false
envelope_to_add = true
return_path_add = true
file = /var/spool/mail/${local_part}
but mutt tells me that /var/spool/mail/paul is not a mailbox.
BTW the way where is local_part defined? Thanks for any more help, Paul