Following message came to me:

550-5.7.26 Unauthenticated email from yahoo.de is not accepted due to domain's 550-5.7.26 DMARC policy. Please contact the administrator of yahoo.de domain if 550-5.7.26 this was a legitimate mail. Please visit 550-5.7.26 https://support.google.com/mail/answer/2451690 to learn about
the 550 5.7.26 DMARC initiative. c23si2057099wmb.180 - gsmtp (in reply to end of DATA command)

This is the mail system at host fastmail.meinestadt.de.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
kontakt2003@meinestadt.de: host aspmx.l.google.com[74.125.140.26] said:
550-5.7.26 Unauthenticated email from yahoo.de is not accepted due to domain's 550-5.7.26 DMARC policy. Please contact the administrator of yahoo.de domain if 550-5.7.26 this was a legitimate mail. Please visit 550-5.7.26 https://support.google.com/mail/answer/2451690 to learn about
the 550 5.7.26 DMARC initiative. c23si2057099wmb.180 - gsmtp (in reply to end of DATA command)