question on ptr rr

[garrett.allen () comcast net]

this may be deemed off topic - if so apologies in advance. however i respect many of the opinions i see here so thought 
i would take a chance and ask. 

we are a stub network, injesting about 30k emails daily.  about a year ago we implemented a spam filtering product.  it 
works well.  recently we turned on the knob to enable it to do reverse lookups.  only the mild version, a reverse is 
made on the ptr rr for the ip address sending the email.  if it fails the spam filter issues a 421 and closes the 
connection.  unfortunately, we have 6 sites thus far that are legitimately trying to communicate with us but don't have 
ptr's associated with the ip address sending emails.  since it obviously isn't a requirement to have one is it 
generally accepted to do so?  any sense for how many end networks do and don't?

thanks in advance.