Outlook deletes emails?

I saved ~ 1000 emails on my laptop, when I logged in to another laptop (older Outlook) the emails were loaded on this laptop and are no longer visible on the Main. What can I do?

Familiarize yourself with the topics POP3 and IMAP as retrieval logs for email.

I guess you have on the "other" laptop, the emails retrieved via POP3 protocol and therefore on the first laptop via "IMAP" (logically) no longer access, as POP3 deletes the mail on the server when retrieved.
IMAP does not do this.

That fixes the problem, thanks. Is there any way to make the mail available on my main?

Export of mails to a PST file, integration of these on the other device.

Theoretically yes, because Outlook has saved the data locally.

I would recommend to backup the system partition of the "other" laptop beforehand - if something goes wrong, the data would still be there.

After that:
In addition to the POP3 mailbox, add the same mailbox with IMAP, so in Outlook the mailbox is now available twice. (Stop once with POP3 and once with IMAP)

Then you can transfer the emails via the POP3 mailbox into the IMAP mailbox via Copy & Paste and upload it back to the server.

Granted, I've never done this before - and would have liked to, before I describe it that way. That's why the proposal for a backup.

Goes too, but that would be the data still local.
If the mailboxes are to be queried with several devices (eg also via smartphone), the data must be uploaded again via the IMAP protocol or another import / export function.

Thank you, 100% security you never have with online answers but with the backup is it anyway safe.