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?