Thank you for your reply.
Indeed, the missing mails appear when I deactivate the cache mode in Outlook. If I then reactivate it, the mails that I sent with Apple Mail for MacOS are missing again in Outlook's "Sent Items". *)
Strange problem. Is there any way to fix this?
*) This cannot be tested in Apple Mail (iOS), as there is no mode without caching. In any case, the mails sent by Apple Mail (MacOS) and stored in the "Sent Items" folder in Apple Mail (iOS) are not displayed.