I do not know how they handle each account holder's address! What I do know is that my UK banks, credit cards, mobile phone (SIM) etc are all registered at the address they have provided for me to use. It cost £12 /month plus fees for actions they take like forwarding Cards etc!
WRT to letters etc they scan it (the first page if a big document) and you decide if they should destroy it, scan the whole thing or send the original by DHL!
PS; You do not need to provide the Mail company your actual permanent address, if you have one! Just tell them for each mail forwarding what address to use; ideal if like me your job took you all over the world at short notice!