[imp] Problems configuring a default domain

[Andy Theuninck]

Versions:
CentOS 5.7
Horde 3.3.11
Imp 4.1.6
PHP 5.1.6
dovecot 1.0.7
postfix 2.3.3

I'm having lots of trouble with Imp using 'example.com' as a default
domain. I can't find any trace of that string in the actual code using
find+grep (outside of PHP comments) nor can I figure out why my domain
settings aren't being applied. I'm hoping for some hint as to what
I've overlooked.

imp/config/servers.php looks like this:
$servers['imap'] = array(
    'name' => 'IMAP Server',
    'server' => 'localhost',
    'hordeauth' => false,
    'protocol' => 'imap/ssl/novalidate-cert',
    'port' => 993,
    'maildomain' => 'mydomain.com',
    'smtphost' => 'localhost',
    'smtpport' => 25,
    'realm' => '',
    'preferred' => '',
);

The comments in that file imply 'maildomain' is the setting I need,
but when I compose a message @example.com is appended in the 'Default
identity' listing.

I've also tried modifying $_prefs['mail_domain']['value'] in
imp/config/prefs.php to my own domain (default was empty string). That
doesn't seem to make any difference.

I'm also having problems with expanding bare addresses. If I compose a
message and just put 'user' for a To address, the message gets sent to
user at example.com. I thought I found the setting for this with
$conf['compose']['add_maildomain_to_unexpandable'] in
imp/config/conf.php, but changing that from true to false doesn't
change the behavior. Ideally I'd like to leave those addresses without
a domain and let postfix sort out to do with them, but failing that
I'd like to at least control what domain gets appended.

I feel like I have to be missing something blindingly obvious, but I'm
at a loss what it could be.

I have verified that the directory where Imp writes conf.php when I
use the Admin => Setup => Generate Mail Configuration is the same
directory where my servers.php and prefs.php files are (and same exact
file with regard to the expansion setting), so I don't think it's a
matter of simply altering the wrong files.

If it's relevant, I have Horde IMAP authentication without a domain.