In-box

How do you tell some one that you cannot mail them because their in-box is full?.....

Check their profile. See when they signed on last. If they have not signed on in a long time, you can consider them inactive. If they have been here recently, maybe they just don't want to answer any messages.

But the mail I write comes back that the party's box is full

They have to log in here before they can get to their mailbox, so if they haven't logged in, they will never see their mailbox.

I recall you had a thread where you tried to contact this particular person, but that thread is no longer available. I checked their profile, and they had not been on in a VERY long time.