Final answer:
Under federal process service rules, legal documents cannot simply be left in a mailbox; they must be delivered to either the individual directly or someone at their residence of suitable age and discretion.
Step-by-step explanation:
Under federal rules for process service, specifically Rule 4 of the Federal Rules of Civil Procedure, the papers cannot simply be left in a mailbox at the residence if no one is home. The rules generally require that the documents be delivered directly to the individual. However, if personal delivery is not possible, the documents may be left at the person's residence or usual place of abode with someone of suitable age and discretion who resides there. Therefore, leaving papers in a mailbox would not meet the requirements of valid service of process under these rules.