is rejecting SMTP traf from unresolv hosts legitim/usefull?

hi, my postfix is configured to reject incommimg traffic from unknown hosts. I found rejected traffic from an ip, which resolves to an official name, at least with nslookup <ipnum>. now when I do a reverse lookup, nslookup <hostname> it says domain not found. hostname beginns with digits and is therefore not rfc conform (?). there is no point in rejecting mail from users (customers), with the message "hey, you got no DNS entry" when they come the other day telling me they got one (but not rfc conform), and if I dont accept it, they change to some other at the far end. but I do not want to relay spammers... what is the background, and should I do something about it? thanks, lars