Seems to be a problem when not defining email's sender address.

strato-production.yaml

@@ -59,9 +59,10 @@
 
         - name: Send Email
           community.general.mail:
-            host: mail.steinle-computer.de
+            host: mail.vodafone.de
             port: 465
-            username: kontakt@steinle-computer.de
+            from: christiansteinle@arcor.de
+            username: christiansteinle@arcor.de
             password: "{{ mail_password }}"
             to: "{{ item.value.email }}"
             subject: Datenbank Backup