Gotchas for upgrading R4 servers to R5.08

I'm in the process of upgrading my two R4 SMTP servers to R5.08. We are a mixed R4/R5 environment. Any gotchas I should look out for? I don't plan on upgrading the design of my other R4 Address Books to R5.
You can peruse the chats I had on migrating to R5. They are on this site in the chat archives. I would suggest getting out of the mixed server environment ASAP. Message fidelity gets trashed a lot when passing between servers on different versions.

This was first published in February 2002