Neomail
To me, I have to have neomail. Some people block it off to friends only, but I can't do that. Someone may want to trade, or I write to them (I write to about 25 people a day, minimum). I can't have only friends mailing me back, in fact, most people I talk to, I won't talk to them ever again.
I need neomail to haggle. In a trade, someone may want you to offer just a little more, then they'd accept. Or someone might see you win an auction and give you a good offer. Or just random people may see you, see your trades, and want to discuss something. You can always delete neomails, but you want to see what people are offering.
Remember, you won't get neomail banned from neomails you get. Only from the amount you send. You can always erase bad neomails, but a lot of people won't request to be a friend if you have "neofriends only". If you blocked that too . . . well, they have no way to contact you, which is good for privacy. It's bad for business though.
Try to keep your inbox clear though, cause you don't want to miss an opportunity cause you had 100 messages cluttering your box, and no one could send you anything.