Some resource limits for sendmail cannot be changed by systemd configuration

Issue

• Some resource limits to sendmail cannot be changed by systemd configuration.

  Output example:

  # cat /etc/systemd/system/sendmail.service.d/limits.conf
  [Service]
  LimitNOFILE=50000
  
  # systemctl daemon-reload
  # systemctl show sendmail.service |grep LimitNOFILE
  LimitNOFILE=50000
  
  # systemctl status sendmail
  ● sendmail.service - Sendmail Mail Transport Agent
     Loaded: loaded (/usr/lib/systemd/system/sendmail.service; enabled; vendor preset: disabled)
    Drop-In: /etc/systemd/system/sendmail.service.d
             └─filelimit.conf
     Active: active (running) since Mon 2018-02-05 10:26:39 JST; 3min 1s ago
    Process: 12287 ExecStart=/usr/sbin/sendmail -bd $SENDMAIL_OPTS $SENDMAIL_OPTARG (code=exited, status=0/SUCCESS)
    Process: 12281 ExecStartPre=/etc/mail/make aliases (code=exited, status=0/SUCCESS)
    Process: 12279 ExecStartPre=/etc/mail/make (code=exited, status=0/SUCCESS)
   Main PID: 12289 (sendmail)
     CGroup: /system.slice/sendmail.service
             └─12289 sendmail: accepting connections
  
  Feb 05 10:26:39 rhel7 systemd[1]: Starting Sendmail Mail Transport Agent...
  Feb 05 10:26:39 rhel7 sendmail[12289]: starting daemon (8.14.7): SMTP+queueing@01:00:00
  Feb 05 10:26:39 rhel7 systemd[1]: PID file /run/sendmail.pid not readable (yet?) after start.
  Feb 05 10:26:39 rhel7 systemd[1]: Started Sendmail Mail Transport Agent.
  
  # cat /proc/12289/limits |grep open
  Max open files            1024                 1024                 files   
  

  In this example, systemd tries to set NOFILE to 50000. But the actual resource limit stays 1024 from process's limit information.