The following Multi verifier will search only ldap://1.2.3.4 when the query is user@a.com. It will search only ldap://1.2.3.5 when the query is user@c.com. It will search first ldap://1.2.3.4. Then, if no match is found, it will search ldap://1.2.3.5 when the query is user@b.com.
<Vrfy version="101.4635">
<Multi>
<InterEnumTime>50</InterEnumTime>
<LDAP defaults="Generic">
<Domain>a.com</Domain>
<Domain>b.com</Domain>
<Host>1.2.3.4</Host>
</LDAP>
<LDAP defaults="Generic">
<Domain>b.com</Domain>
<Domain>c.com</Domain>
<Host>1.2.3.5</Host>
</LDAP>
</Multi>
</Vrfy>