You can simply utilize the
MacMister Mac Apple Mail Converter Software. It is the best software to
migrate Apple Mail to Office 365 account and other file formats like Yahoo, Gmail, Thunderbird, IMAP, PST, DOC, MBOX, MSG, HTML, EML, etc. Data reliability and original folder hierarchy are preserved throughout the migration process. Moreover, there is a free demo edition of this utility that provides migration of the first 25 Apple mail emails per folder. So, utilize this free software to perform the Apple Mail file conversion.