[Exim] Backup mail server configs

Is anyone using exim as a main server as well as 1 (or more) exim
servers as a backup MX? If so, can someone explain how to configure it
properly?

I have the backup simply able to relay messages for the
domains that are hosted on the main mail server, and it properly
accepts and queues messages. However, it seems that the backup server
ignores any retry rules listed and trys to send the messages
immediatly, resulting in a failed message because of "a suspected mail
loop."