[Dovecot] Can Dovecot authenticate against an external email server?

Just wondering if anyone has done this.

I have a spam filtering service where I am now storing spam for users I filter for. It's a filter and forward service so I don't control the recipient's email server.

What I would like to do somehow is have the user enter their email address and password and then look up their POP/IMAP server from a database and try to authenticate from it. If sucessful then the user will be able to access their stored spam using Dovecot and Squirrelmail.

Any suggestions?

Thanks in advance