Error Message "Failure to Send (12)"

I am trying to send a message on an account, but I get the error message "Failed to send. (12)"
What is going on and how do I correct it? I cannot send or reply on this account.
I have 2 other accounts and they work OK, but this one will not let me send a message or reply back.
It says it is already processing a message and cannot send another now when I try to reply.

I found the problem -- my Friend List was maxed out.