$HOME/.mh_profile The user's profile.
So, we will use $HOME if it is set.
/var/mail/$USER Location of the system mail drop.
But we don't actually use $USER (we call getpwuid(getuid()) and use that).
I personally interpreted the use of $USER as "the username goes here",
not "we use the value of the $USER environment variable". But I admit
that this is not clear.
But I don't understand why it didn't just follow unix conventions as
most users of MH have been on Unix.
See my previous email; turns out that it was a bug.