What are some alternatives?
When comparing mnIMAPSync and Piler, you can also consider the following products
imapsync - Console-based utility for migrating IMAP mailboxes.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Yippie Move - The world's best email transfer tool
Gmvault - Backup the emails in your Gmail account.
OfflineIMAP - OfflineImap synchronizes email between an IMAP server and a MailDir or between two IMAP servers.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.