Looks like inc pays attention to $MAILDROP and if it is not set and
profile entry MailDrop is not set, it looks into /var/mail/$USER.
That's pretty much right.
Not sure if it ever checks $MAIL or $MAILPATH.
It doesn't, and it doesn't use $USER, or $LOGNAME, either.
If the environment variable $MAILDROP is set, then inc uses it as
the location of the user's mail drop instead of the default (the
-file name switch still overrides this, however). If this
environment variable is not set, then inc will consult the profile
entry “MailDrop” for this information. If the value found is not
absolute, then it is interpreted relative to the user's nmh
directory. If the value is not found, then inc will look in the
standard system location for the user's mail drop.
The source says
MAILDROP environment variable.
maildrop profile entry.
mmdfldir/mmdflfil from mts.conf.
mmdfldir defaults to HOME environment variable.
mmdflfil is ‘unknown’ if getpwuid(3) fails or there's no
pw_name, else it's the user part of the profile's optional
local-mailbox, else it's pw_name from getpwuid(3).
Martin, along with all the other suggestions, could you be a mix of
users thanks to su(1) and thus your shell is telling you of mail in one
location, but inc(1) looking in another?