Make tells only possible after attaining level 10-15 or make busy mode have adjustable options so you can allow friend list or FC to /tell you etc.

That or add a permissions list that you can add to just like a friends list so they may /tell you even when you're in busy mode.

As for mailing, make it so you can send things to FC members as well rather than just people in your friends list. Sure you could easily stick it in the chest if you have access but not everyone does.

I also know you could just make sure the other person is right next to the chest to prevent theft but even so that's just more work than needed. Especially huge FC's where the possibility for a jerk to snatch it when reserved for someone else is quite possible so this would null that ability to happen.