problem with keystrokes in email

I have a client, that when he replies to an email, and tries to insert text, the system inserts his name in brackets ie) [user name] and I can find no reason why, and it seems to be random. If I go to the top of the email, sometimes I can start typing, and then go to where I want and insert text, but then when I try and go to the next section to type, the same event happens!!!

Weird, never seen anything like this, any idea?