i search for an App, that could acces serveral Email Accounts via POP3/IMap, and Download the Mails to Nextcloud, so that i could word with my Outlook, phone… directly my nextcloud via imap?
Nextcloud is not a mail/IMAP server. It has a mail client.
As @KarlF12 has already written Nextcloud provides an email client only. The desired function can only be provided by a function running on your email server, by e.g. using Fetchmail on it.