Email rejected because of @ sign outside of an email address

[sturzhelmut]

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