MTA operators, please remember that a lack of PTR records may cause false positives in regard to spam filtering.
As RFC 1912 put it, back in 1996:
Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain. If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a (sic) alias defined by a CNAME.
Obviously, if a mail admin is not competent enough to set up reverse DNS, the outside world will assume that his machine is not legitimately intended to serve mail, but rather is being abused by a botnet operator or similar villain.