@web.de and @gmx.de respond with "550-Requested action not taken: mailbox unavailable 550 invalid DNS MX or A/AAAA resource record"

Question

I'm running a webserver with Debian 10 (the provider is 1blu) on it. To send emails by php sendmail I installed postfix and configured it according to the description from 1blu.

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# See http://www.postfix.org/COMPATIBILITY_README.html -- default to 2 on
# fresh installs.
compatibility_level = 2



# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mail.tracemyrace.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, localhost.localdomain, v81017.1blu.de, localhost.1blu.de, localhost
relayhost = 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
smtp_bind_address = 178.XXX.XX.XX #IP address of my tld

When I send an email to e.g. @t-online.de or @googlemail.com everything works fine. But mails to web.de and gmx.de get bounced:

May  1 18:30:17 v81017 postfix/pickup[12809]: 2277AE08FC: uid=33 from=<www-data>
May  1 18:30:17 v81017 postfix/cleanup[13352]: 2277AE08FC: message-id=<[email protected]>
May  1 18:30:17 v81017 postfix/qmgr[595]: 2277AE08FC: from=<[email protected]>, size=462, nrcpt=1 (queue active)
May  1 18:30:17 v81017 postfix/smtp[13354]: 2277AE08FC: to=<[email protected]>, relay=mx-ha02.web.de[212.227.17.8]:25, delay=0.04, delays=0/0/0.03/0.01, dsn=5.0.0, status=bounced (host mx-ha02.web.de[212.227.17.8] said: 550-Requested action not taken: mailbox unavailable 550 invalid DNS MX or A/AAAA resource record (in reply to MAIL FROM command))
May  1 18:30:17 v81017 postfix/cleanup[13352]: 2F424E08FD: message-id=<[email protected]>
May  1 18:30:17 v81017 postfix/bounce[13365]: 2277AE08FC: sender non-delivery notification: 2F424E08FD
May  1 18:30:17 v81017 postfix/qmgr[595]: 2F424E08FD: from=<>, size=2601, nrcpt=1 (queue active)
May  1 18:30:17 v81017 postfix/qmgr[595]: 2277AE08FC: removed
May  1 18:30:17 v81017 postfix/local[13366]: 2F424E08FD: to=<[email protected]>, relay=local, delay=0.02, delays=0/0.01/0/0.01, dsn=2.0.0, status=sent (delivered to command: procmail -a "$EXTENSION")
May  1 18:30:17 v81017 postfix/qmgr[595]: 2F424E08FD: removed

I have an reverse-entry at 1blu. And a lookuptool show this result: mxtoolbox results

Could this result explain the problem I have? In that case I wonder why the problem only occurs with this providers. Maybe someone could help me how to find the error or what I have to configure different? I would appreciate that, it's my first question here, for all others in the past I found the solution by web search. But in this case I don't have a clue what I could do.