this is driving me up the wall
On mobile devices there are different settings:
Select the correct encryption: SSL/TLS
i am not using a mobile device.
Did you try setting up to the IONOS account with a different mail client as a test?
Unfortunatelly there are no information about your configuration as the Support Template asked you to fill at thread-creation. The also are questions concerning entries (error messages) in the log files (Nextcloud.log, Apache/Nginx access.log etc.
Do you have access to the IONOS mail logs? Maybe there can be found the reason, why connection has been rejected.
okay I have found the issue, DigitalOcean dont allow use to SMTP ports.