How does GWIA check valid addresses?

We use MessageLabs to screen inbound email and they report increased
volumes looking like dictionary spam attacks. MessageLabs have asked us to
provide, and keep up-to-date, a list of valid email addresses so they can
ignore emails without valid recipients instead of processing them and our
GWIA drops them.

Nicknames give valid smtp addresses, resource addresses can be valid and
Distribution list addresses can be valid but none of them are available to
an LDAP lookup. Exporting the address book will include resource and
distribution list addressez but still misses Nicknames and mises any
addresses which are hidden from the Address Book.

How does the GWIA check for valid email addresses? Is there any way we can
get into that routine to extract a full list of valid addresses? Is there
any other way we can get a full list of valid addresses out of Groupwise?

Anthony