pop3 reverse proxy with authentication

Hello,

We are using exchange 2010 SP2.

We have some external users which use now pop3 to download their email messages from our exchange 2003.

I need to put a solution in place so these users can be migrate to exchange 2010.

I'm looking for some reverse proxy which also does ldap authentication.

A cas server in dmz is not supported so that is not an option for me, also looked at forefront 2010 but no authentication for pop3.

Anyone any recommendation what I can use?

Thanks for the help.

Best regards,

JB