我已经在严重故障上发布了这个问题，但是当我提到我的服务器上安装了控制面板（即使它与问题无关）时，我的问题就被关闭了。
这里是：
我有一台 Centos 6.5 VPS（带有 ISPConfig 面板），最近尝试为我的电子邮件通信安装 SSL/TLS 证书（该证书是标准的 RapidSSL 证书）。
我对一个问题感到困惑，Dovecot 可以很好地接受我的 SSL 证书，并允许我毫无问题地接收电子邮件，但 Postfix 拒绝连接（我在 ThunderBird 中收到的消息是：“无法发送消息，因为连接到传出）服务器 (SMTP) mail.example.com 失败。服务器可能不可用或拒绝发送服务器 (SMTP) 连接，请验证您的发送服务器 (SMTP) 设置是否正确，然后重试”。
Postfix 可以在端口 587 上与 STARTTLS 和普通身份验证配合使用，但不能与端口 465 上的 SSL/TLS 配合使用。
我是电子邮件系统的新手，因此我完全不确定可能导致此问题的原因。
这是我的 postfix 的 master.cf：
smtp inet n - n - - smtpd
submission inet n - n - - smtpd
# -o smtpd_tls_security_level=encrypt
# -o smtpd_tls_security_level=may
# -o smtpd_sasl_auth_enable=yes
# -o smtpd_client_restrictions= permit_sasl_authenticated, reject
# -o milter_macro_daemon_name=ORIGINATING
# smtps inet n - n - - smtpd
# -o smtpd_tls_wrappermode=yes
# -o smtpd_sasl_auth_enable=yes
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject
# -o milter_macro_daemon_name=ORIGINATING
#628 inet n - n - - qmqpd
pickup fifo n - n 60 1 pickup
cleanup unix n - n - 0 cleanup
qmgr fifo n - n 300 1 qmgr
#qmgr fifo n - n 300 1 oqmgr
tlsmgr unix - - n 1000? 1 tlsmgr
rewrite unix - - n - - trivial-rewrite
bounce unix - - n - 0 bounce
defer unix - - n - 0 bounce
trace unix - - n - 0 bounce
verify unix - - n - 1 verify
flush unix n - n 1000? 0 flush
proxymap unix - - n - - proxymap
proxywrite unix - - n - 1 proxymap
smtp unix - - n - - smtp
# When relaying mail as backup MX, disable fallback_relay to avoid MX loops
relay unix - - n - - smtp
-o smtp_fallback_relay=
# -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
showq unix n - n - - showq
error unix - - n - - error
retry unix - - n - - error
discard unix - - n - - discard
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - n - - lmtp
anvil unix - - n - 1 anvil
scache unix - - n - 1 scache
maildrop unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/bin/maildrop -d ${recipient} ${extension} ${recipient} ${user} ${nexthop} ${sender}
dovecot unix - n n - - pipe
flags=DROhu user=vmail:vmail argv=/usr/libexec/dovecot/deliver -f ${sender} -d ${user}@${nexthop}
amavis unix - - - - 2 smtp
-o smtp_data_done_timeout=1200
-o smtp_send_xforward_command=yes
127.0.0.1:10025 inet n - - - - smtpd
-o content_filter=
-o local_recipient_maps=
-o relay_recipient_maps=
-o smtpd_restriction_classes=
-o smtpd_client_restrictions=
-o smtpd_helo_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,reject
-o mynetworks=127.0.0.0/8
-o strict_rfc821_envelopes=yes
-o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
amavisfeed unix - - n - 2 lmtp
-o lmtp_data_done_timeout=1200
-o lmtp_send_xforward_command=yes
-o disable_dns_lookups=yes
-o max_use=20
这是我的 Postfix main.cf
sendmail_path = /usr/sbin/sendmail.postfix
# newaliases_path: The full pathname of the Postfix newaliases command.
# This is the Sendmail-compatible command to build alias databases.
#
newaliases_path = /usr/bin/newaliases.postfix
# mailq_path: The full pathname of the Postfix mailq command. This
# is the Sendmail-compatible mail queue listing command.
#
mailq_path = /usr/bin/mailq.postfix
# setgid_group: The group for mail submission and queue management
# commands. This must be a group name with a numerical group ID that
# is not shared with other accounts, not even with the Postfix account.
#
setgid_group = postdrop
# html_directory: The location of the Postfix HTML documentation.
#
html_directory = no
# manpage_directory: The location of the Postfix on-line manual pages.
#
manpage_directory = /usr/share/man
# sample_directory: The location of the Postfix sample configuration files.
# This parameter is obsolete as of Postfix 2.1.
sample_directory = /usr/share/doc/postfix-2.6.6/samples
# readme_directory: The location of the Postfix README files.
#
readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES
virtual_alias_domains =
virtual_alias_maps = hash:/etc/mailman/virtual-mailman, proxy:mysql:/etc/postfix/mysql-virtual_forwardings.cf, proxy:mysql:/etc/postfix/mysql-virtual_email2email.cf
virtual_mailbox_domains = proxy:mysql:/etc/postfix/mysql-virtual_domains.cf
virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql-virtual_mailboxes.cf
virtual_mailbox_base = /var/vmail
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_sasl_authenticated_header = yes
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, check_recipient_access mysql:/etc/postfix/mysql-virtual_recipient.cf
smtpd_tls_auth_only = no
smtpd_tls_security_level = may
# smtpd_tls_cert_file = /etc/postfix/smtpd.cert
# smtpd_tls_key_file = /etc/postfix/smtpd.key
smtpd_tls_cert_file = /usr/certs/mail_xxxx_xx.pem
smtpd_tls_key_file = /usr/certs/mail_xxxx_xx.key
transport_maps = hash:/var/lib/mailman/data/transport-mailman, proxy:mysql:/etc/postfix/mysql-virtual_transports.cf
relay_domains = mysql:/etc/postfix/mysql-virtual_relaydomains.cf
relay_recipient_maps = mysql:/etc/postfix/mysql-virtual_relayrecipientmaps.cf
proxy_read_maps = $local_recipient_maps $mydestination $virtual_alias_maps $virtual_alias_domains $virtual_mailbox_maps $virtual_mailbox_domains $relay_recipient_maps $relay_domains $canonical_maps $sender_canonical_maps $recipient_canonical_maps $relocated_maps $transport_maps $mynetworks
smtpd_sender_restrictions = check_sender_access mysql:/etc/postfix/mysql-virtual_sender.cf
smtpd_client_restrictions = check_client_access mysql:/etc/postfix/mysql-virtual_client.cf
smtpd_client_message_rate_limit = 100
maildrop_destination_concurrency_limit = 1
maildrop_destination_recipient_limit = 1
virtual_transport = dovecot
header_checks = regexp:/etc/postfix/header_checks
mime_header_checks = regexp:/etc/postfix/mime_header_checks
nested_header_checks = regexp:/etc/postfix/nested_header_checks
body_checks = regexp:/etc/postfix/body_checks
smtp_tls_security_level = may
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_protocols = !SSLv2,!SSLv3
smtp_tls_protocols = !SSLv2,!SSLv3
myhostname = mail.xxxx_xx
mynetworks = 127.0.0.0/8 [::1]/128
#mynetworks = 178.43.213.42/8 [::1]/128
dovecot_destination_recipient_limit = 1
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
content_filter = amavis:[127.0.0.1]:10024
receive_override_options = no_address_mappings
relayhost =
mailbox_size_limit = 0
message_size_limit = 0
如果有人可以帮助解决这个恼人的问题，我将非常感激。
更新：（我还不能对此堆栈交换站点发表评论）。我的postfix版本是2.6.6
update2：即使我取消注释 master.cf 文件中的 smtps 块，连接仍然被拒绝。
答案1
您必须取消smtps注释中的块main.cf，然后重新启动 Postfix。 STARTTLS 之所以有效，是因为它使用标准smtp端口 (25)。
答案2
如果您的计算机上安装了任何防火墙，则必须向该防火墙添加端口规则。防火墙示例：iptables,ufw
大多数时候，开发人员配置了 dovecot 和 postfix 等邮件服务器，但他们忘记为25、143、587、等端口添加规则993。
要启用 UFW 上的端口：
ufw status // this will list all current rules
ufw allow [port] //For example: ufw allow 25
向 iptables 添加规则：
//SMTP
sudo iptables -A INPUT -p tcp --dport 25 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 25 -m conntrack --ctstate ESTABLISHED -j ACCEPT
//IMAP
sudo iptables -A INPUT -p tcp --dport 143 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 143 -m conntrack --ctstate ESTABLISHED -j ACCEPT
//IMAPS
sudo iptables -A INPUT -p tcp --dport 993 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 993 -m conntrack --ctstate ESTABLISHED -j ACCEPT
//POP3
sudo iptables -A INPUT -p tcp --dport 110 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 110 -m conntrack --ctstate ESTABLISHED -j ACCEPT
//POP3S
sudo iptables -A INPUT -p tcp --dport 995 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 995 -m conntrack --ctstate ESTABLISHED -j ACCEPT