我在一台新的 Ubuntu 24.04 服务器上安装了 Postfix。如果我从本地远程登录到它，它会按预期以 SMTP 横幅响应。但是，如果我从外部源远程登录到它，它会打开端口，然后什么也不做，即我收到一个空白屏幕，并且对于我发送的任何命令（helo、ehlo、helo mail.test.com 等），都会出现“452 语法错误（连接）”错误。显然，这不是防火墙问题，因为 postfix 会回答，它似乎只是不想为来自外部网络的请求提供服务。
我检查了所有的配置，并重启了服务，甚至重启了机器。我不知道是什么导致了这个问题。
/etc/postfix/main.cf
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
readme_directory = no
compatibility_level = 3.6
# TLS parameters
smtpd_tls_cert_file = /etc/nginx/ssl/************/2275563/server.crt
smtpd_tls_key_file = /etc/nginx/ssl/************/2275563/server.key
smtpd_tls_security_level = may
smtp_tls_CApath=/etc/ssl/certs
smtp_tls_security_level = may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = ************
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = virtuallyanything.com, $myhostname, svr-virtuallyanything.localdomain, localhost.localdomain, localhost
relayhost = smtp.postmarkapp.com:2525
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
virtual_mailbox_domains = pgsql:/etc/postfix/sql/pgsql_virtual_domains_maps.cf
virtual_alias_maps = pgsql:/etc/postfix/sql/pgsql_virtual_alias_maps.cf, pgsql:/etc/postfix/sql/pgsql_virtual_alias_dom>
virtual_mailbox_maps = pgsql:/etc/postfix/sql/pgsql_virtual_mailbox_maps.cf, pgsql:/etc/postfix/sql/pgsql_virtual_alias>
virtual_transport = lmtp:unix:private/dovecot-lmtp
smtp_tls_note_starttls_offer = yes
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_local_domain =
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
milter_protocol = 6
milter_mail_macros = i {mail_addr} {client_addr} {client_name} {auth_authen}
milter_default_action = accept
smtpd_milters = inet:127.0.0.1:11332
non_smtpd_milters = inet:127.0.0.1:11332
/etc/postfix/master.cf
#
# Postfix master process configuration file. For details on the format
# of the file, see the master(5) manual page (command: "man 5 master" or
# on-line: http://www.postfix.org/master.5.html).
#
# Do not forget to execute "postfix reload" after editing this file.
#
# ==========================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (no) (never) (100)
# ==========================================================================
smtp inet n - y - - smtpd -v
#smtp inet n - y - 1 postscreen
#smtpd pass - - y - - smtpd
#dnsblog unix - - y - 0 dnsblog
#tlsproxy unix - - y - 0 tlsproxy
#127.0.0.1:submission inet n - y - - smtpd
#submission inet n - y - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
# -o smtpd_tls_auth_only=yes
# -o local_header_rewrite_clients=static:all
# -o smtpd_reject_unlisted_recipient=no
# Instead of specifying complex smtpd_<xxx>_restrictions here,
# specify "smtpd_<xxx>_restrictions=$mua_<xxx>_restrictions"
# here, and specify mua_<xxx>_restrictions in main.cf (where
# "<xxx>" is "client", "helo", "sender", "relay", or "recipient").
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
# -o smtpd_helo_restrictions=
# -o smtpd_sender_restrictions=
# -o smtpd_relay_restrictions=
# -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
# -o smtpd_helo_restrictions=
# -o smtpd_sender_restrictions=
# -o smtpd_relay_restrictions=
# -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
-o milter_macro_daemon_name=ORIGINATING
# Choose one: enable submissions for loopback clients only, or for any client.
#127.0.0.1:submissions inet n - y - - smtpd
#submissions inet n - y - - smtpd
# -o syslog_name=postfix/submissions
# -o smtpd_tls_wrappermode=yes
# -o smtpd_sasl_auth_enable=yes
# -o local_header_rewrite_clients=static:all
# -o smtpd_reject_unlisted_recipient=no
# Instead of specifying complex smtpd_<xxx>_restrictions here,
# specify "smtpd_<xxx>_restrictions=$mua_<xxx>_restrictions"
# here, and specify mua_<xxx>_restrictions in main.cf (where
# "<xxx>" is "client", "helo", "sender", "relay", or "recipient").
# -o smtpd_client_restrictions=
# -o smtpd_helo_restrictions=
# -o smtpd_sender_restrictions=
# -o smtpd_relay_restrictions=
# -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
# -o milter_macro_daemon_name=ORIGINATING
#628 inet n - y - - qmqpd
pickup unix n - y 60 1 pickup
cleanup unix n - y - 0 cleanup
qmgr unix n - n 300 1 qmgr
#qmgr unix n - n 300 1 oqmgr
tlsmgr unix - - y 1000? 1 tlsmgr
rewrite unix - - y - - trivial-rewrite
bounce unix - - y - 0 bounce
defer unix - - y - 0 bounce
trace unix - - y - 0 bounce
verify unix - - y - 1 verify
flush unix n - y 1000? 0 flush
proxymap unix - - n - - proxymap
proxywrite unix - - n - 1 proxymap
smtp unix - - y - - smtp
relay unix - - y - - smtp
-o syslog_name=postfix/$service_name
# -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
showq unix n - y - - showq
error unix - - y - - error
retry unix - - y - - error
discard unix - - y - - discard
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - y - - lmtp
anvil unix - - y - 1 anvil
scache unix - - y - 1 scache
postlog unix-dgram n - n - 1 postlogd
#
# ====================================================================
# Interfaces to non-Postfix software. Be sure to examine the manual
# pages of the non-Postfix software to find out what options it wants.
#
# Many of the following services use the Postfix pipe(8) delivery
# agent. See the pipe(8) man page for information about ${recipient}
# and other message envelope options.
# ====================================================================
#
# maildrop. See the Postfix MAILDROP_README file for details.
# Also specify in main.cf: maildrop_destination_recipient_limit=1
#
#maildrop unix - n n - - pipe
# flags=DRXhu user=vmail argv=/usr/bin/maildrop -d ${recipient}
#
# ====================================================================
#
# Recent Cyrus versions can use the existing "lmtp" master.cf entry.
#
# Specify in cyrus.conf:
# lmtp cmd="lmtpd -a" listen="localhost:lmtp" proto=tcp4
#
# Specify in main.cf one or more of the following:
# mailbox_transport = lmtp:inet:localhost
# virtual_transport = lmtp:inet:localhost
#
# ====================================================================
#
# Cyrus 2.1.5 (Amos Gouaux)
# Also specify in main.cf: cyrus_destination_recipient_limit=1
#
#cyrus unix - n n - - pipe
# flags=DRX user=cyrus argv=/cyrus/bin/deliver -e -r ${sender} -m ${extension} ${user}
#
# ====================================================================
#
# Old example of delivery via Cyrus.
#
#old-cyrus unix - n n - - pipe
# flags=R user=cyrus argv=/cyrus/bin/deliver -e -m ${extension} ${user}
#
# ====================================================================
#
# See the Postfix UUCP_README file for configuration details.
#
uucp unix - n n - - pipe
flags=Fqhu user=uucp argv=uux -r -n -z -a$sender - $nexthop!rmail ($recipient)#
# Other external delivery methods.
#
#ifmail unix - n n - - pipe
# flags=F user=ftn argv=/usr/lib/ifmail/ifmail -r $nexthop ($recipient)
#bsmtp unix - n n - - pipe
# flags=Fq. user=bsmtp argv=/usr/lib/bsmtp/bsmtp -t$nexthop -f$sender $recipient
#scalemail-backend unix - n n - 2 pipe
# flags=R user=scalemail argv=/usr/lib/scalemail/bin/scalemail-store ${nexthop} ${user} ${extension}
#mailman unix - n n - - pipe
# flags=FRX user=list argv=/usr/lib/mailman/bin/postfix-to-mailman.py ${nexthop} ${user}
尽管我最初认为这是防火墙问题。根据 FedKad 的评论，VPS 提供商将连接发送到死胡同，尽管他们的文档只说他们阻止了传出端口 25，而不是传入端口。