[exim] Hostnames

Hi

Is there a valid reason for a mail server to connect to my mailserver from
the same ip with different hostnames (as told in helo/ehlo)?
I am thinking about blacklisting ips that tell me more then 3 hostnames from
the same ip within less than 24 hours for about a week.

Every legal mailserver I know always connects using the same helo name.
But a lot of spammers connect multiple times using different helo names from
the same ip.

Any thoughts on this?

Regards,
Steffen