[exim] U=hidden-user need understanding

Hi all

I seem to be seeing something in my logs

U=hidden-user

actual line

2006-05-18 10:29:10 H=(cpt.proteahotels.com) [66.8.31.51] U=hidden-user F=<karen@???> temporarily rejected RCPT <myuser@???>: host lookup deferred for reverse lookup check

I googled for "U=hidden-user" but alas nothing comes up.

If someone could please help me undertand this, I would be most grateful

Kind Regards
Brent Clark

P.s. Heres the best part

dig cpt.proteahotels.com +short
192.168.1.5