The best and the reliable method to backup emails of IMAP-supported email clients is by opting for a professional software approach. However, I have gone through an interesting and informative writeup that helped me to migrate IMAP emails to Office 365 manually. I have decided to go through this approach because I was already aware of this process and I just wanted a brief introduction to this process.
However, this article explained the detailed manual approach in a simpler manner to avoid any data loss. This manual approach uses the Office 365 Admin Center to migrate the IMAP emails. Additionally, to make the process easier this article discussed a professional IMAP Migration Tool.
This software is a professional approach to migrating IMAP emails into various email clients. Additionally, it can help users to backup the emails of IMAP-supported email clients in different file formats. You can try its free version to analyze the performance of this advanced utility.