Yum-cron 无法向 root 发送电子邮件

我正在尝试调查我们的一台 CentOS 7 服务器上的一个问题，该服务器yum-cron无法通过执行操作的结果向 root 发送电子邮件。

它总是给出这个错误：

无法向 localhost 发送电子邮件：[Errno 111] 连接被拒绝

但是，我们还有其他具有相同配置的服务器，这是唯一出现此类问题的服务器。

以下是yum-cron.conf内容：

[commands]
#  What kind of update to use:
# default                            = yum upgrade
# security                           = yum --security upgrade
# security-severity:Critical         = yum --sec-severity=Critical upgrade
# minimal                            = yum --bugfix update-minimal
# minimal-security                   = yum --security update-minimal
# minimal-security-severity:Critical =  --sec-severity=Critical update-minimal
update_cmd = default

# Whether a message should be emitted when updates are available,
# were downloaded, or applied.
update_messages = yes

# Whether updates should be downloaded when they are available.
download_updates = yes

# Whether updates should be applied when they are available.  Note
# that download_updates must also be yes for the update to be applied.
apply_updates = yes

# Maximum amout of time to randomly sleep, in minutes.  The program
# will sleep for a random amount of time between 0 and random_sleep
# minutes before running.  This is useful for e.g. staggering the
# times that multiple systems will access update servers.  If
# random_sleep is 0 or negative, the program will run immediately.
# 6*60 = 360
random_sleep = 360


[emitters]
# Name to use for this system in messages that are emitted.  If
# system_name is None, the hostname will be used.
system_name = None

# How to send messages.  Valid options are stdio and email.  If
# emit_via includes stdio, messages will be sent to stdout; this is useful
# to have cron send the messages.  If emit_via includes email, this
# program will send email itself according to the configured options.
# If emit_via is None or left blank, no messages will be sent.
emit_via = email

# The width, in characters, that messages that are emitted should be
# formatted to.
output_width = 80


[email]
# The address to send email messages from.
email_from = root

# List of addresses to send messages to.
email_to = root

# Name of the host to connect to to send email messages.
email_host = localhost


[groups]
# NOTE: This only works when group_command != objects, which is now the default
# List of groups to update
group_list = None

# The types of group packages to install
group_package_types = mandatory, default

[base]
# This section overrides yum.conf

# Use this to filter Yum core messages
# -4: critical
# -3: critical+errors
# -2: critical+errors+warnings (default)
debuglevel = -2

# skip_broken = True
mdpolicy = group:main

# Uncomment to auto-import new gpg keys (dangerous)
# assumeyes = True

我仔细检查过，它与其他服务器上的配置文件相同。

此外，所有服务器都postfix安装为邮件服务器，使用 sendgrid 作为 smtp 中继。

最后，在所有mail服务器上，通过命令手动向 root 发送电子邮件可以正常工作。

我应该检查什么才能使 cron 正确地向 root 发送电子邮件？

编辑：

经过一些测试，我注意到在有问题的服务器上，没有监听 TCP 端口 25：

[root@srv1 ~]# ss -tnlp | grep :25
[root@srv1 ~]#

在另一台服务器上，我得到：

[root@srv2 ~]# ss -tnlp | grep :25
LISTEN 0 100 127.0.0.1:25 *:* users:(("master",pid=768,fd=13))
[root@srv2 ~]#

PID 768 的进程在哪里/usr/libexec/postfix/master -w。

然后，我检查了哪些进程对postfix服务处于活动状态，并在我得到的第一台服务器上：

[root@srv1 ~]# service postfix status
Redirecting to /bin/systemctl status  postfix.service
● postfix.service - Postfix Mail Transport Agent
   Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
   Active: active (running) since Tue 2017-01-10 09:18:55 CET; 5min ago
  Process: 17409 ExecStop=/usr/sbin/postfix stop (code=exited, status=0/SUCCESS)
  Process: 17431 ExecStart=/usr/sbin/postfix start (code=exited, status=0/SUCCESS)
  Process: 17428 ExecStartPre=/usr/libexec/postfix/chroot-update (code=exited, status=0/SUCCESS)
  Process: 17421 ExecStartPre=/usr/libexec/postfix/aliasesdb (code=exited, status=0/SUCCESS)
 Main PID: 17503 (master)
   CGroup: /system.slice/postfix.service
           ├─17503 /usr/libexec/postfix/master -w
           ├─17504 pickup -l -t unix -u
           └─17505 qmgr -l -t unix -u

而在第二个输出是：

[root@srv2 ~]# service postfix status
Redirecting to /bin/systemctl status  postfix.service
● postfix.service - Postfix Mail Transport Agent
   Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2016-12-28 16:34:19 CET; 1 weeks 5 days ago
 Main PID: 768 (master)
   CGroup: /system.slice/postfix.service
           ├─ 768 /usr/libexec/postfix/master -w
           ├─ 770 qmgr -l -t unix -u
           ├─8185 pickup -l -t unix -u
           └─9148 tlsmgr -l -t unix -u

所以看起来第一台服务器tlsmgr根本没有启动，但是我在日志中找不到任何错误。