And what happens if you try to telnet to Sendmail on port 25?
If there are reverse dns lookup problems, you will be able to read the offending mail addresses in the /var/log/maillog file.
If you logon to the server and do "dig -t MX " for a valid mail address domain, this will test the reverse lookup connection from the mail server.
eg dig -t MX yahoo.com will give you the ip number of the yahoo mail servers. (man dig for more options)
If that email is coming from a different source address, why would you want to receive it?