In what cirstances can the post office open your mail without a warrant?

The source of the obscene material may have been an undercover agent, and a warrant may have been obtained using the testimony of the undercover agent. There may have also been a tip from a reliable source regarding the content. I suspect the postal inspector did have a warrant and the police who followed him home had a separate warrant to search the house for more obscene material or related items connected to the case.