If they are using html emails or tag in their reply, it will simply CHOP the message at the illegal tag..
Believe me, I've been hunting through the code to "Allow" certain users to send html messages, but so far, no joy.. we get orders in HTML table format, which without the code are impossible to read! (all the numbers are immediately adjacent, so you can't tell what the quantity is etc)
If I figure it out, I'll post here.. but for now, you could run the input through htmlentities first, so that it doesn't b0rk and renders the input directly, rather than parsing it as code.