Check your DNS configuration:
https://intodns.com

Check your MX configuration:
https://mxtoolbox.com

Check your HELO/EHLO:
https://www.abuseat.org/lookup.cgi

Check your mail server IP listings on:
https://www.spamhaus.org/lookup/

Online Terminal-Tools:
https://toolbox.googleapps.com/apps/dig/
https://www.digwebinterface.com

QUELLE 2020v06: https://www.gmx.net/mail/senderguidelines/