How to sync messages to all resources of the same user

account1@example.com/resourcea
account2@example.com/resourceb
account2@example.com/resourcec
account2@example.com/resourced

when account1 send message to account2
account2@example.com/resourceb ,account2@example.com/resourcec ,account2@example.com/resourced can receive the message.

reference http://www.ejabberd.im/node/4994

My question is when account2@example.com/resourceb send a message to account1
how account2@example.com/resourcec and account2@example.com/resourced sync the message from account2@example.com/resourceb

sos~