isync (mbsync) is described as 'mbsync is a command line application which synchronizes mailboxes. New messages, message deletions and flag changes can be propagated both ways; the operation set can be selected in a fine-grained manner' and is an app in the Backup & Sync category. There are five alternatives to isync (mbsync) for Windows, Linux, Mac, iPhone and BSD. The best alternative is MailStore. It's not free, so if you're looking for a free alternative, you could try imapsync or OfflineIMAP. Other great apps like isync (mbsync) are mnIMAPSync (Free) and Softaken IMAP to IMAP Migration (Paid).
Console-based utility for migrating IMAP mailboxes. imapsync software is a command line tool allowing incremental and recursive imap transfers from one mailbox to another, both anywhere on the internet or in your local network.
OfflineImap synchronizes email between an IMAP server and a MailDir or between two IMAP servers. It offers very powerful and flexible configuration options, that allow things such as the filtering of folders, transposing of names via static configuration or python scripting.
mnIMAPSync - Java IMAP email syncing tool mnIMAPSync can be used to migrate or backup an IMAP account to another either in the same server or in different servers. The program can be run standalone (command line interface) or accessed directly from java.
Softaken IMAP to IMAP Migration allows transfer of emails from one IMAP Server to another. All IMAP Servers are compatible with this application. Users can easily migrate complete mailbox folders between IMAP Server accounts with attachments.