Pop3 and LDAP authentication...Multiple radius servers

Hi,    We have radius server which is inhouse which does the LDAP authentication. We got a new request from third party to do authentication for "their" users using POP3. So the request comes to radiusA (our inhouse radius). If the user has realm as @xyz.net ..then we forward the request to third party to authenticate which might be radiusB which does the authentication using POP3. If there is no realm attached, radiusA does the LDAP auth and return the response. Not sure how to specify in our radiusd.conf. I could not find any thread in the list. Please let me know the link if this is already discuss. Really Appreciated your quick response. Thanks and Regards.