Mail screening is the physical screening of post/mail prior to reaching its final destination.
This may be from a postroom/mailroom or from an external premise through a professional mail screening provider.
Typically this is a security function of an organisation and differing levels of security are required for mail screening depending on the risks associated with/to the organisation.
A specification for mail screening and security (PAS 97) provides relevant information on what is important for an organisation.
Professional security consultants will have an in depth knowledge of these processes.