In ar_mailer 1.5.1, running as a daemon, the pid file is not written.

eg:
 ar_sendmail -d -p tmp/ar_sendmail.pid

Expect pidfile to be written, but it is not.

I've only quickly looked at the code, but it looks like WEBrick::Daemon.start is called before check_pid is called.
Perhaps moving the WEBrick stuff to just after check_pid would fix it.

Only a minor issue, but having the pid file is a nice way to reliably stop the process.