nginx is a high performance smtp/pop/imap proxy that lets you do custom authorization and lookups and is very scalable. (just add nodes) Nginx by default is a reverse proxy and this is what it is doing here for pop/imap connections. It is also an excellelent reverse proxy for web servers. Advantage: You dont have to have a speacial database or ldap schema. Just an url to do auth and lookup with. A url that may be accessed by a unix or a tcp socket. Write your own auth handler - according to your own policy. For example: A user called atif tries to login with the pass testxyz. You pass this infomation to a URL such as socket:/var/tmp/xyz.sock or http://auth.corp.mailserver.net:someport/someurl The auth server replies with either a FAILURE such as Auth-Status: Invalid Login or password or with a success such as Auth-Status: OK Auth-Server: OneOfThe100Servers Auth-Port: optionalyAPort We have implemented it at our ISP and it has saves us a lot of headaches. This would work for both imap and pop. I have not yet tried for SMTP.