[Exim] dhcp-hosts in reject_hosts

Hello all,

Does anybody know whether it's possible to place dhcp-hosts in the
reject_hostst file, of another way to solve this?
The problem is that I recieve a rather large amount of virusses from a
sender who hasn't got a static IP#, but recieves one through a
dhcp-server from his ISP. Although he hasn't got a static IP#, he has
got a hostname like user###.cable.isp.com. Is it possible to reject mail
from this user-domain?

Ofcourse I would not want to ban all users of this ISP and the
virus-sender does not send the virusses through his own e-mail address,

Thanks in advance for any answers.
With kind regards,

Thijs