[Dovecot] Dovecot Authentication through different IMAP server?

Just wondering - is there any way for a dovecot server to authenticate against a different IMAP server?

Here's what I'm thinking. I run a spam filtering service and thinking about a way to give users access to their stored spam on my servers. However real email is forwarded to their server, So I'm wondering if I can configure dovecot somehow to do a back end authentication to their email account to gain access to their spam.

Open to suggestions