Reverse lookups in sendmail

Hi, Any ideas of how to avoid having sendmail trying to do reverse lookup on hosts trying to relay through it? Cause when hosts on my network want to send an email, since I dont have bind installed and we use the DNS servers at our ISP, it takes alot for the MUA to finish talking to the MTA(since you have to wait until the reverse lookup fails (sendmail tries at least 4 times before quiting) and then it sends your email). Thanks, Raul