Chapter 500 of the MPEP deals with the receipt and handling of mail and papers when received by the USPTO.
This can included from applicants as well as from third parties. Additionally, this chapter discusses what forms of communication are allowed and under what circumstances. For example, when can facsimile be used to file documents? or when is it okay to use a certificate of mailing? There are usually a few questions on the patent bar exam regarding these procedures so care should be taken when reviewing the material. Make special note of the proper use of facsimile and the filing date afforded certificate of mailings and where they can be mailed from.