Exim and backup relays

[Longman]

Ever since setting up a backup MX for my domain (via relay domains) I've
started receiving lots of Frozen Message emails for Frozen Emails in the
queue on this backup MX. It looks like spammers are using the the backup
MX to send email to non-existant recipients (computer generated nonsense
in most cases), but of course they're accepted because their all to
accounts within my domain that I've told it to relay for.  What is the
correct way to try and combat this? Is there a way to get the backup
relay to contact the primary relay to get a list of valid email accounts
thereby junking nonsense recipients, or some other method to put paid to
this?  Currently I have 9468 Frozen Messages!