One of the companies I work for has a set if servers behind their Firewall.

The firewall has openings for web, ftp, and email access to those servers from the outside world. The company hosts their own site and mail servers. They have a DSL connection to the Internet, and have an ISP pointing their DNS entries to the boxes for them.

Periodically, the mail server will slow to a crawl where mail sending is concerned.

I've run a tcpdump capture of traffic and the majority of the traffic appears to be reverse DNS lookups for the server's private IP address (192.168.x.x).

After running the tcpdump utility for just a few minutes (maybe 5 or so), I see that the filter captured 49,267 packets. But it reports that the kernel dropped 4598 packets.

Where can I look to find the cause of this problem?

I'd check the mailservers' hostfile for having it's own hostname defined, resolv.conf for where the resolvers point to and the LAN's DNS server for providing correct local net records. Would be easy to set up a caching DNS (Pdnsd?) to supply info and help tweak lookup performance.