Alright, it is working and here is what I did but do not ask me what the effect did:
First I added to my config file:
EMAIL_TRANSPORT_DEFAULT_HOST: "mail.instagmbh.net"
EMAIL_TRANSPORT_DEFAULT_PORT" "587"
EMAIL_TRANSPORT_DEFAULT_USERNAME: "my username"
EMAIL_TRANSPORT_DEFAULT_PASSWORD: "mypasswor"
EMAIL_TRANSPORT_DEFAULT_TLS: "true"
EMAIL_DEFAULT_FROM: "no-reply@instagmbh.net"
After that, no change!!!
Then in the Gui I did:
Email provider → Other
Authentication method → Username & password
Username → myemail@instagmbh.net
Password → mypassword
Advanced settings
SMTP host → mail.2606.eu (that ist the domain my mailserver runs on)
Use TLS → Yes
Port → 587
SMTP client → instagmbh.net
Sender configuration
Sender name → no-reply
Sender email → no-reply@instagmbh.net
Bam it was running.
I will try to limit the problem within the next few days and let you know. For now I decided not to talk to the functioning email Thanks to all for the great support