Sendmail无法找到收件人导致邮件发送失败的排查求助

我正在尝试在服务器上配置sendmail来发送告警通知，查了不少网上的指南后，还是没法把外发邮件送到我的工作邮箱。根据我看到的资料，sendmail不需要额外配置就能发邮件，我已经检查了25端口、DNS配置，也修改了主机名，但都没成功。下面是我收到的退信：

From: Mail Delivery Subsystem MAILER-DAEMON@company.com
Message-Id: 202307050151.3651ovxS001207@company.com
To: root@company.com
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="3651ovxS001207.1688521876/company.com"
Subject: Returned mail: see transcript for details
Auto-Submitted: auto-generated (failure)

This is a MIME-encapsulated message

--3651ovxS001207.1688521876/company.com

The original message was received at Wed, 5 Jul 2023 01:50:57 GMT
from root@localhost

----- The following addresses had permanent fatal errors -----
staff.recipient@company.com
(reason: 550 5.1.1 staff.recipient@company.com... User unknown)
(expanded from: staff.recipient@company.com)

----- Transcript of session follows -----
... while talking to [127.0.0.1]:

DATA
<<< 550 5.1.1 staff.recipient@company.com... User unknown
550 5.1.1 staff.recipient@company.com... User unknown
<<< 503 5.0.0 Need RCPT (recipient)

--3651ovxS001207.1688521876/company.com
Content-Type: message/delivery-status

Reporting-MTA: dns; company.com
Arrival-Date: Wed, 5 Jul 2023 01:50:57 GMT

Final-Recipient: RFC822; staff.recipient@company.com
Action: failed
Status: 5.1.1
Remote-MTA: DNS; [127.0.0.1]
Diagnostic-Code: SMTP; 550 5.1.1 staff.recipient@company.com... User unknown
Last-Attempt-Date: Wed, 5 Jul 2023 01:51:16 GMT

--3651ovxS001207.1688521876/company.com
Content-Type: message/rfc822

Return-Path:
Received: (from root@localhost)
by company.com (8.15.2/8.15.2/Submit) id 3651ovxR001207
for staff.recipient@company.com; Wed, 5 Jul 2023 01:50:57 GMT
Date: Wed, 5 Jul 2023 01:50:57 GMT
From: root
Message-Id: 202307050150.3651ovxR001207@company.com
Subject: Test email

Test email from linode

--3651ovxS001207.1688521876/company.com--

从退信里能明显看到，sendmail一直在尝试连接本地的127.0.0.1投递邮件，还把staff.recipient@company.com当成了本地用户，但服务器上根本没有这个用户，所以才会报“User unknown”的错误。给你几个关键的排查方向：

• 检查本地域名配置：打开/etc/mail/local-host-names文件看看，如果里面有company.com这条记录，sendmail会默认把这个域名当作本地域名，所有发往该域名的邮件都会优先尝试在本地服务器找用户，而不是转发到外部邮件服务器。如果有这个条目，把它注释掉或者删除，然后重启sendmail服务：systemctl restart sendmail（或者service sendmail restart，看你的系统版本）。

• 配置智能主机（Smart Host）：如果你们公司有专门的邮件服务器，应该让sendmail通过这个服务器转发所有外发邮件。编辑sendmail的m4配置文件（一般是/etc/mail/sendmail.mc），添加或修改这一行：

  define(`SMART_HOST',`mail.company.com')dnl
  

  把mail.company.com换成你们公司实际的邮件服务器地址，然后运行m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf生成新的配置文件，最后重启sendmail服务。

• 验证MX记录解析：用dig company.com MX命令检查company.com的MX记录是否正确指向了你们公司的邮件服务器。如果MX记录配置错了，sendmail找不到正确的投递目标，就会转而尝试本地投递。

• 查看详细日志：去/var/log/maillog或者/var/log/mail.log里找sendmail的详细日志，里面会记录更完整的投递过程，能帮你确定到底是DNS问题、配置问题，还是网络连接的问题。

另外还要提一句，有些云服务商（比如你用的Linode）可能默认封禁了25端口，需要你在控制台申请解封，这也会导致外发邮件失败，这个点也可以排查下。

备注：内容来源于stack exchange，提问作者user6354238