Apparently every email sent from Odoo has two sender fields (one in the header, one in the envelope) . The header contains the address of the employee who sent the mail, e.g. max.meyer@xyz.de. So far ok.

In the envelope, however, the address of the Odoo mail system is listed, in the example max.meyer@xyz.com.

This leads to our mails getting stuck in numerous spam filters with the following Error message: MAIL FROM domain (xyz\.com\)\ and\ Header\-From\ domain\ \(xyz.de) are not equal.