A few words about the internal design.

Iris is multi-threaded at its core. Both networking and file operations like attachment handling are all done on their own processes. Each network connections has its own, so sending and receiving from multiple mailboxes can all happen at the same time.

The lowlevel email handling is based on a commercial library designed with IMAP in mind. In fact, Iris is mostly IMAP oriented, with minimal support for POP3 servers.

Because of that, there's currently no way of importing emails from other clients - there's no local storage for emails. At least, not yet.