BB Software :: Emails Being Sent In Charset Gb18030?

I'm a System Administrator with a company based in Singapore. We use Blackberries of various model on the StarHub network. We don't use a BES server, the devices pull email via IMAP from our company email servers (BIS)Emails sent by some users are being encoded in the gb18030 charset, despite the text of the email, and the language setting of the device itself, both being in English. The body of the mail is presented in Base64.These emails appear blank when viewed under Outlook 2003, and presumably other clients that haven't Chinese fonts installed.Sample headers of a test mail are:

Content-Transfer-Encoding: base64
Subject: Test
To: pgargan@mycompany.com
From: "User" <user@mycompany.com>

[code]....

Why is RIM / StarHub encoding English-language emails as gb18030? More importantly, how can I disable this behaviour and instead get it to use a charset such as "Windows-1252"?