Dovecot can act as IMAP and POP3 proxy forwarding user transparently to a specified mailbox server.
With proxy feature you can have
- Multiple mailbox hosts with single IMAP/POP3 host address for your users.
- SSL and TLS support for possible existing infrastructure lacking those.
- IPv6 to IPv4 gateway for existing infrastructure lacking IPv6 support.
Make sure that the capabilities you are advertising with dovecot are also supported by the imap server you are proxying to, else IMAP clients might generate errors.