They are too numerous and vast to mention here. The main ones are the hundreds of bug fixes and major enhancements from R5 to ND6.5. You should check the release notes and fix lists databases on Notes.net for items that pertain to your situation.
Dig Deeper on Domino Resources - Part 7
Related Q&A from Michael Lazar
Learn how to easily keep track of the numbers of incoming and outgoing Lotus Notes email messages to and from your Domino server each day. Continue Reading
Discover what steps you can take if your Lotus Domino server runs out of disk space and you have issues sending and receiving Lotus Notes email ... Continue Reading