[Dovecot] lda and home directory

[Bob Miller]

Hello,

I am using qmail and lda configured such that lda should not have to do
a lookup for delivery.  I set my defaultdelivery like so:

|HOME=/home/mail/$USER /var/qmail/bin/preline
-f /usr/local/libexec/dovecot/dovecot-lda

Given that the email address being delivered to is
bob.miller at computerisms.com, I expect $USER to be equal to bob.miller
and $HOME to expand to /home/mail/bob.miller/.  

The problem is lda reports:

Debug:_Home_dir_not_found:_/home/mail/bob.miller at computerisms.com/

The whole system up to here uses only the bob.miller part, and I can't
find where it keeps adding the domain part from.

I have tried changing all sorts of variables and config values to try
and make it quit adding the domain part, but no matter what I change lda
still tries to deliver the email address as username.  The only measure
of success I can report is that in some configurations mail delivers
to /home/mail//.

Where exactly is dovecot getting this value?

Side question: I note in places throughout the wiki and internet
variables such as $EXT, $USER, $DEST_USERNAME, and $FROM_ENVELOPE are
used.  I can find no documentation, beyond my own common sense anyway,
to explain how/when these variables are expanded, and what I do find
doesn't seem consistent to me.  For example, on the CheckPassword page,
$USER is implied to expand to Username, which could contain the domain
or not.  However on the lda/qmail page, the variable shown is $EXT@
$USER, which implies $USER will expand as only the domain part after the
@ symbol and $EXT as everything before, yet changing the variable from
$USER to $EXT in my defaultdelivery file has $HOME expand
to /home/mail//.  Where do I find the story on how these variables (as
opposed to the % variables) work?  (or more specifically, is there a
$VAR I can use instead of $USER that will expand to just the part before
the @?)

-- 
Computerisms
Bob Miller      
867-334-7117 / 867-633-3760
http://computerisms.ca