How do I have to implement this so that it also works on the Syncloud server?
"Nextcloud mails are not displayed - solved
What has now helped is to delete the account from the database.
If you only have one account, then it is ID=1 in the database. I
deleted it using the MySQL command, but you can also delete it directly in the table using phpMyAdmin:
DELETE FORM oc_mail_accounts WHERE id=1