Suppose that we have an organization with legacy e-mail server that supports IMAP protocol hosted with different company.
We know only email login information, e.g. IMAP host, users and their respective passwords.
Step 1. - Collecting and verifying information.
We need to check if the IMAP login information is correct.
Then we setup same account to the new e-mail server with the same password. We also install some web based interface like RoundCube that accesses the new e-mail server.
Step 2. - Initial migration of emails.
All data is migrated. This step is done by us. During the migration, all e-mails are accessible without any interruption from the old server in normal way.
After the migration is complete, each user can log into RoundCube and check if everything is transferred correctly. This is optional since there is a log file for the migration and all errors should be logged.
Step 3. - Update MX records and change of the e-mail server.
When the organization is ready to proceed with change of the e-mail server, first we do migration of all emails received from the initial migration.
Then we change MX records of the domain names and update any records like 'mail.domain.com' if any.
All users update their e-mail information in their E-Mail readers such as Outlook, Apple Mail, Firebird etc.
Note that for the next 2-3 days, the old e-mail server still might receive some emails. To avoid any email loss, we setup a continuous migration of any new emails that might come to the old server.
Finally we can remove all e-mails from the old e-mail server.
By using such a setup, the organization can change their e-mail server, with some small e-mail delays, but without any e-mail downtime.
In the same way we can back up any existing e-mail servers.