To migrate IMAP emails to other file extension, webmail apps, you can get the CubexSoft IMAP Backup tool (https://www.cubexsoft.com/imap-backup/) that can simply transfer IMAP files to other file formats, webmail apps. It is simple to access for everyone. The tool supports multiple IMAP accounts at once. Using the tool, you can export IMAP data to PST, PDF, MBOX, IMAP Server, EML, EMLx, Gmail, Office 365, Exchange Server, Outlook.com, Yahoo Mail, etc. https://www.cubexsoft.com/blog/import-imap-into-office-365/