It is recommended to perform email migration as per steps given below:

1) Create the email addresses in RediffmailPro server.
2) To prevent any kind of data loss it is suggested to point the domain MX record to RediffmailPro (new mails will come on Rediffmailpro & old data can be migrated).
3) Access the Migration tool, input user credentials and IMAP server details to start the process and you can login with the admin email address ie; admin@yourdomain.com via ‘Admin’ tab to monitor the migration status.

Note: There can be some security settings applied by your existing service provider which might not allow the utility to connect using IMAP. In such cases, it is suggested to disable or do low level settings for such security features.
For eg: You need to turn on ‘less secure apps’ settings as Gsuite domain Administrator to allow data migration from Gsuite to Rediffmailpro.