error when getmail calls maildrop

[rlharris@oplink.net]

This post to debian-user concerns the same problem which I described
in a recent post to the getmail list.  This posting provides greater
detail.

System:
    i386
    Debian 8 (Jessie, xfce)
    getmail 4.46.0-1
    maildrop 2.7.1-3

I am attempting to re-create a mailsystem which was running
successfully several years ago under Debian Etch.  My goal is for
getmail to download messages from a POP3 server (using
SimplePOP3Retriever) and pass each message to maildrop.  Running in
delivery mode, maildrop is to categorize the message and deposit it
in the corresponding maildir.

THE PROBLEM: getmail sees the messages but cannot pass them to
maildrop.  The getmail log reports an error, but nothing is written to
the maildrop log.  The log files are extant; they are located in:
~/mail/logs/getmail ~/mail/logs/maildrop.

The error message is:
----------------------------
2015-08-01 04:07:47 Delivery error (command maildrop 3808 error (75,
/usr/bin/maildrop: Unable to open mailbox.))

2015-08-01 04:07:47 msg 2/1373 (16327 bytes) msgid 000007da53c4329f
from <SRS0+e50bb6a0873f79ec=B6=cs.foobar.com=bounce@mx25.net>

2015-08-01 04:07:47 2 messages (47056 bytes) retrieved, 0 skipped
----------------------------

The set of maildirs is extant; they were created with the command:
    mkdir -vp /path/new-maildir-name/{cur,new,tmp}

As a test, I changed the [destination] from
----------------------------
    type = MDA_external
    path = /usr/bin/maildrop
    unixfrom = true
----------------------------
to
----------------------------
    type = Maildir
    path = /home/rh/maildir-test/
----------------------------
With this change, message delivery takes place without error.

File permissions on getmail and maildrop are as left by synaptic:
----------------------------
    -rwxr-xr-x 1 root root 39558 Apr 22  2014 /usr/bin/getmail
    -rwxr-sr-x 1 root mail 219164 Jan 14  2015 /usr/bin/maildrop

The maildrop configuration file is ~/.mailfilter .  File permissions
on .mailfilter are:
----------------------------
    -rw------- 1 rh rh 14729 Aug  1 05:38 .mailfilter

The getmail script is being run by a normal user rh.  Adding the user rh
to the mail group did not solve the problem.

RH