Hi Paul,
Yes, I thought it is some script you're having problem with.
I have not tried to sort out/research this issue but I feel:
a) mail to the userID (cpanel login) @ domain will have this tag (the mails are apparently going to hostmaster account of the domain
root@harrier.xxxx..xxx)
b) email Ids created within cpanel should not have this extra line
You could try changing the domain hostmater (SOA records) email just to test.
Regds
IJ