In the past couple days, some emails from my server have been bouncing with this error. Nothing has changed on my end that I'm aware of. It doesn't seem to matter where the email is going-- they bounce whether the destination is Gmail, Hotmail, or other smaller domains.
As far as I can tell my server is not any spam blacklists like this one. It's a dedicated server that I control completely. I don't think there is any anti-spam software running on it except fail2ban, which shouldn't affect email.
The drops seem to be caused by the content of the emails. This answer suggests a man in the middle attack, probably my own hosting provider stopping the emails. But they are encrypted, so I thought this shouldn't be possible.
Here is my main.cf file:
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu) biff = no append_dot_mydomain = no readme_directory = no smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination myhostname = server1.myserver.com alias_maps = hash:/etc/aliases alias_database = hash:/etc/aliases myorigin = /etc/mailname mydestination = $myhostname, server1.myserver.com, localhost.myserver.com, myserver.com, mail.myserver.com, localhost virtual_alias_domains = domain1.com, domain2.com.com virtual_alias_maps = regexp:/etc/postfix/virtuala-regex, hash:/etc/postfix/virtualalias relayhost = mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 mailbox_size_limit = 0 recipient_delimiter = + inet_interfaces = all # The following line isn't in the Ubuntu example file inet_protocols = all # TLS smtp_tls_security_level = may smtpd_tls_security_level = may smtp_tls_note_starttls_offer = yes smtpd_tls_key_file = /etc/ssl/ssl.key/myserver.com.key smtpd_tls_cert_file = /etc/ssl/ssl.crt/myserver.com.crt smtpd_tls_CAfile = /etc/ssl/ssl.crt/myserver.com.ca-bundle smtpd_tls_loglevel = 1 smtpd_tls_received_header = yes smtpd_tls_session_cache_timeout = 3600s tls_random_source = dev:/dev/urandom # DKIM milter_default_action = accept milter_protocol = 2 smtpd_milters = inet:localhost:12999 non_smtpd_milters = inet:localhost:12999 message_size_limit = 52428800
Am I accidentally giving my hosting provider permission to snoop on emails with one of my postfix settings?