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

vars/strato-production.yaml

@@ -5,11 +5,11 @@ mysql_databases: ['buehler', 'scheidle', 'roundcube', 'svj']
 local_backup: /home/semaphore/backup
 mail_password: !vault |
   $ANSIBLE_VAULT;1.1;AES256
-  61643461323362653230613639343261623939393031333835613234373037386439613431303833
+  62626138363135383764313264386136666666303466363864663739643462643533313339616431
-  3233393366333038653033613036656266646539613666370a353964333739636233666532616531
+  3264306564313461663132623333333065316533653462660a623336346239663964316431356336
-  32303537393035386466366261353061646563666565353462393864653338383661333639336665
+  38653661336165653261386532633430616165353731366234313365376134343461373763363065
-  3837663733396433360a303736363366653161313062393434623265653464643930623732356561
+  6637653437656138620a653563626237366633313035323337626232636537313866626365313931
-  6463
+  6135
 buehler-password: !vault |
   $ANSIBLE_VAULT;1.1;AES256
   37306230376433313631326230313739396238666239323362366164363831643664306535346464