But the default security model should be enough in your scenario. Each user's mail file has only the user's name and their home server in the ACL of that database. Any user trying to access information will be rejected. If you set the default access wide open, then anyone can access your mail file. For public mail-in databases, this is often done.
Notes has many security layers.
With that, you can throw in form and view level security and the abiltiy to encrypt this information at any level. Man in the middle attacks of this sort will only work if the targeted database in question already allows the attacking user access. First of all, a few words from me. Really poor implementations of good algorithms. He is using Lotus Domino client to access his corporate e-mail account.