This parameter for the mail_replica plug-in indicates the location for replication. This example gives a sense of its use:
mail_replica = remote:firstname.lastname@example.org
When a new mail message is saved via IMAP or a message is being delivered via LDA/LMTP, the system waits this amount of time for the mail to be synced to the remote site. If it doesn’t finish after this timeout, a status of success is indicated anyway.
replication_sync_timeout = 2s