In an EasyDiscuss setup where the forum is public and users can email to a specific address, you are not going to know who is going to email you. A whitelist in this case is not useful as you don't know who is going to email you.
However you may want to block certain senders and this can be achieved with a blacklist. This allows to selectively exclude users who you do not want their emails processed.
Having both lists gives us end-users the best of both worlds.
If this cannot be implemented as such, a plugin trigger would also be useful, I can write a plugin that hooks into it then.