I haven’t had any feedback on this issue; if this has been forgotten about feel free to just let me know
I have found out that the user causing the issue in my case uses the webmail client for “BT Mail”.
BT Mail is the optional free email service that comes with BT Broadband. When you register for BT Broadband you’ll be asked if you’d like a BT Mail address to use. If you choose this option you’ll then be given an email address that ends in @btinternet.com.
EDIT:
Not sure how many users they have now in 2015 - but I’m thinking it’s a few:
BT Group operates the BT Total Broadband brand and had 4.6 million accounts in January 2009.
This issue is on my list, but low priority for now. Also, this issue is most probably due to bug in GitHub email_reply_parser library, so I will have to fix it upstream.