最近又碰到一起Linux下SendMail發送郵件失敗的案例，郵件發送后，郵箱收不到具體郵件， 查看日志/var/log/maillog 發現有"DSN: User unknown"以及“dsn=5.1.1, stat=User unknown”等錯誤信息，脫敏后的具體日志如下所示：
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnA017770: Authentication-Warning: mylnx01.xxxx.xxxx.com: oracle set sender to mylnx01-RMAN@xxxxx.com using -f
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnA017770: from=mylnx01-RMAN@esquel.com, size=17698, class=0, nrcpts=2, msgid=<201710311645.v9VGjfnA017770@mylnx01.xxxx.xxxx.com>, relay=oracle@localhost
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ2017771: <xxxxxx@esquel.com>... User unknown
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ2017771: <xxxxxx@esquel.com>... User unknown
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnA017770: to=zhangl@xxxxxx.com, ctladdr=mylnx01-RMAN@xxxxxx.com (502/502), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=77698, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnA017770: to=eeldba@xxxxxx.com, ctladdr=mylnx01-RMAN@xxxxxx.com (502/502), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=77698, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ2017771: from=<mylnx01-RMAN@xxxxxx.com>, size=17698, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnA017770: v9VGjfnB017770: DSN: User unknown
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ4017771: <mylnx01-RMAN@xxxxxx.com>... User unknown
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnB017770: to=mylnx01-RMAN@xxxxxx.com, delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=48722, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ4017771: from=<>, size=18722, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnB017770: v9VGjfnC017770: return to sender: User unknown
Nov 1 00:45:41 mylnx01 sendmail[17771]: v9VGjfZ6017771: from=<>, size=22066, class=0, nrcpts=1, msgid=<201710311645.v9VGjfnC017770@mylnx01.xxx.xxxxxx.com>, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Nov 1 00:45:41 mylnx01 sendmail[17770]: v9VGjfnC017770: to=postmaster, delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=49746, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (v9VGjfZ6017771 Message accepted for delivery)
Nov 1 00:45:41 mylnx01 sendmail[17772]: v9VGjfZ6017771: to=root, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=52298, dsn=2.0.0, stat=Sent
出現這個問題的原因是因為在測試sendmail的一個功能時，在配置文件/etc/mail/local-host-names了設置了一些域名，這個配置文件可以用它來實現虛擬域名或多域名支持，一般當主機有多個名稱時，而當你希望多個主機名都能收到郵件時，那么就必須設置這個配置文件。
[root@mylnx01 log]# more /etc/mail/local-host-names
# local-host-names - include all aliases for your machine here.
xxxxx.com
而偏偏這台服務器的hostname跟local-host-names里面指定的不一樣。從而導致郵件發送異常問題。注釋掉這個主機名后，SendMail發送郵件就正常。
[root@mylnx01 log]# more /etc/mail/local-host-names
# local-host-names - include all aliases for your machine here.
#xxxxx.com
參考資料：
https://www.linuxquestions.org/questions/linux-newbie-8/sendmail-error-user-unknown-dsn%3D5-1-1-a-4175460428/
http://linux.vbird.org/linux_server/0380sendmail.php#server_local-host-names