Somehow the "new version" of Outlook for macOS fucked up and all "Sent" Mails we're only saved locally. And turns out, moving them to an IMAP-Folder syncs nothing. Litterly useless. To sync the missing mail to your server, this is the only way I got the mails back in my IMAP/webmail/Apple Mail.
- Create a folder on your desktop like
mails-outlook-export
- Go to the folder in "new" Outlook for Mac, which are not synced
- Mark all the mails with CMD+A
- Move all mails with drag & drop to your folder
mails-outlook-export
- Rename the folder to
mails-outlook-export.mbox
(file extension)