[Exim] host_reject influences DNS-checks?

Dear folks,

here is some problem. Whenever I set a host_reject
configuration-command to block SMTP traffic from a
specific host (e.g. host_reject = 192.168.10.10)
EXIM does also start to check reverse-DNS setups for
every host that connects to the MTA. Why that?

Thanks for your help!

Regards,
Sascha