I am planning to move a website's hosting provider but I am not too sure if I should back up all the existing emails stored locally on the user's machine.
I am being a bit over cautious as I need to migrate to the new server without losing any of the existing emails. Even if they are only stored locally.
In the past moving server caused all emails to disappear on the local machine which makes me think that there is more to it than just changing the DNS.
Mail protocol is IMAP.
Yes and no.
You need to migrate the mailboxes from your existing server to your new server, not from the IMAP clients. In IMAP the server holds the authoritative copy of a mailbox. If a client successfully connects to your server and finds an empty mailbox, it will delete all of its local messages in order to be in the same state as the server. The best way to migrate your mail depends on how it's stored on your server and how seamless you want the transition to be.
