"Private mailboxes" refers to personal, secure mailboxes that individuals or businesses can rent or own to receive their mail privately instead of using a public mailbox or their home address. Full definition
An online private mailbox to enable others to exchange personal messages. (fsat.ru)
There is provision for private mailbox so that the messages that are exchanged would not be shared with public and separate chat rooms as well. (senior-dating-sites.com)
Christian singles enjoy chat rooms, message boards, photo personals, a photo gallery, anonymous private mailboxes, and much more. (searching-hearts.com)