Reportbug ignores smtphost and mta config file settings

I am attempting to use reportbug to report debian bugs from an ubuntu machine from within an office where ports 25 and 587 are blocked, so mail can only go via the configure locahost mail setup.

If I set bts debian in /etc/reportbug.conf (so that I don't get the 'use launchpad for ubuntu bugs' warning), then setting 'smtphost localhost' or 'mta /usr/sbin/sendmail' are both ignored and reportbug always tries to send the mail direct via SMTP to reportbug.debian.org:587, which always fails. I have not managed to work out where this setting is coming from.

If I specify '--smtphost locahost' on the command line, then it works as expected. It really shouldn't be different when this is specified in the config file.

I've search though the code and grepped the files in the reportbug and python-reportbug packages, but I can't see where this override is coming from, nor can I grok where the config file is being read. So I'm at a loss for further debugging.

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: reportbug 6.4.3ubuntu1 [modified: usr/bin/reportbug]
ProcVersionSignature: Ubuntu 3.2.0-35.55-generic 3.2.34
Uname: Linux 3.2.0-35-generic x86_64
ApportVersion: 2.0.1-0ubuntu15.1
Architecture: amd64
Date: Thu Jun 13 16:50:01 2013
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 SHELL=/bin/bash
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 LANGUAGE=en_GB:en
SourcePackage: reportbug
UpgradeStatus: Upgraded to quantal on 2013-04-29 (44 days ago)
mtime.conffile..etc.reportbug.conf: 2013-06-13T16:29:47