[exim] weird line

Hi all

I just found this weird ONE line entry in my maillog:
2005-08-16 09:18:17 H=(mx2.timco.co.za) [196.15.175.140] U=25 F=<> rejected RCPT <866517@???>: Unrouteable address

What interest me is:
U=25

Would anyone be able to help me understand this.

Kind Regards
Brent Clark