There is a huge problem with email lists. People don’t know when they are sending to one. Programs that work with address books don’t know how to recognize them. The end result is that people make mistakes and things get posted to lists (unsubscribes, vacation messages, etc.)
This is a great example of a ‘design hole’. SMTP/RFC822 email is so flexible, it doesn’t differentiate any of the delivery endpoints for email. Unfortunately, this makes it impossible for programs to know that an email address belongs to a list.
IMHO, if I had the chance to do it all over, here is what I would do.
I would make email addresses for lists or any address that the user should be careful of sending to something like this: sales%somecompany.com. That way the user’s email program and any of the programs that used the contacts could easily avoid those addresses or treat them differently.