When hMailServer performs anti-spam tests on a message it will use the connecting IP address to determine where the message is arriving from. When hMailServer receives an email from a MX backup server, hMailServer can't use the senders TCP/IP address since this is the IP address of the backup server. If you add the MX backup servers IP address as an incoming relay, hMailServer will know that messages from this TCP/IP address is being forwarded and that the backup sender is not the original sender. hMailServer will then try to determine the original senders IP address by parsing the Received headers of the email message.
Most anti spam methods in hMailServer are performed before the actual message body has been delivered to hMailServer. hMailServer uses the HELO, MAIL FROM and RCPT TO commands to determine whether the message is likely to be spam. However, if a message arrives from a incoming relay, hMailServer has to wait until the entire message has been received before it can perform anti spam. In other words, using incoming relays will make the anti spam mechanisms more expensive, in terms of bandwidth and time.
Please note that incoming relays does not affect grey listing. Grey listing always takes place before the Received headers have been transmitted to hMailServer. You may want to add any incoming relays IP address to the grey listing white list, to prevent incoming relays from being grey listed.