Email rejected because of @ sign outside of an email address

  • Hello,
    we have problems to send emails to different domains. They are all rejected with the following error:
    550 5.7.1 This email was rejected because it violates our security
    policy The TO header contains an @ sign outside of an email address.

    This error occurs when you are using contacts from your contacts folder. If you write an email address into the To: field everything works fine.
    You can see the reasons why the emails are rejected when you display a contact from your contacts folder. Every contact gets automaticly a display name which contains the email address. Neither it is possible to change the display name nor to delete it.

    How can we solve this problem?

    core: 8.7.9
    webapp: 3.5.14
    deskapp: 2.5.4

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