Redmine

Currently i have a problem with my Redmine docker based installation. I migrated from a turnkey linux appliance to a docker based installation and everything went smoothly. All data and settings migrated.

Now i recognized that Redmine is not sending all Emails to all Users which are observers of a ticket. Some emails are send some not.
When i send myself test emails via the Adminpage this works perfectly and some users always get all emails but some users not. The email adresses in the accounts are correct.
When i look into the container log file, i see the following error messages:

App 26040 output: I, [2019-06-05T08:59:23.832804 #26040]  INFO -- : Performed ActionMailer::DeliveryJob (Job ID: ef69e436-d3c3-48b7-8950-579d1715fdf7) from Async(mailers) in 139.54ms

App 26040 output: E, [2019-06-05T08:59:23.879560 #26040] ERROR -- : Email delivery error: end of file reached

App 26040 output: I, [2019-06-05T08:59:23.879925 #26040]  INFO -- : Performed ActionMailer::DeliveryJob (Job ID: 34073d49-c156-4fd3-8940-59330a7d530b) from Async(mailers) in 244.2ms

App 26040 output: E, [2019-06-05T08:59:24.009490 #26040] ERROR -- : Email delivery error: end of file reached

App 26040 output: I, [2019-06-05T08:59:24.009731 #26040]  INFO -- : Performed ActionMailer::DeliveryJob (Job ID: 44766a58-9215-407b-9958-9b8ceb80ca77) from Async(mailers) in 232.92ms

I have absolutely no idea what causes this at the moment. We are using an internal Exchange-Server which is maintained by an external IT Service company. They tell me everything on the Exchange-Server looks fine. Certificates are new. And that certain emails never are never received by the Server.
So my questions is now if my configuration is correct for exchange:

    delivery_method: :smtp
    smtp_settings:
      enable_starttls_auto: true
      address: "ex-1" 
      port: 587
      domain: "mydomain.de" # 'your.domain.com' for GoogleApps
      authentication: :login
      user_name: "user" 
      password: "password" 

I also tried adding the option openssl_verify_mode: none but this also doesn't help. Does anyone have a hint what i can try ?