Whether or not you will accept mail from an address without a (matching?)
reverse record is usually an option on your mailserver software.
In terms of outbound mail, AOL, for example will no longer accept mail
unless the reverse is present.
Some providers insist that it match, which makes things a little dicey if
your mail server is handling multiple domains. I've gotten around that by
wasting an ip address for each domain, and setting up the reverse records
accordingly. Also, lots of folks are now verifying the validity of the
sender to try to prevent spoofing the mailfrom. (although the spammers only
have to spoof a valid email address to get around that)