how to configure backup MX to relay messages to primary MX

[eli]

Hello,

given a domain has two mx servers:
domain.com.  5  mx1.domain.com.
domain.com.  10 mx2.domain.com.

when someone's delivery agent can't talk to mx1 due to network issues, 
he may retry to deliver messages to mx2.
if mx2 get the messages successfully, how does it relay the mssages to 
mx1 for mail store?

thanks.
Eliza