[xwiki-users] Problems emailing the xwiki user group

Hi, When I email from my browser with my wrong email address I get: You are not allowed to post to this mailing list, and your message has been automatically rejected. If you think that your messages are being rejected in error, contact the mailing list owner at users-owner(a)xwiki.org. When I email from my browser with my right email address the request gets through. But when I email from the email client on my PC (Thunderbird) with my right email address the message neither gets through nor gets bounced. Any ideas why that might be? Could it be that I am emailing in HTML rather than Plain Text format? I wouldn't have thought so. Thanks, Jonathan