For other diagnostic information, check the following sources.
Look at the Received lines in the header of the message. These lines trace the route that the message took as the message was relayed. Remember to consider time–zone differences.
Check the system log that records delivery problems for your group of systems. The sendmail program always records its activities in the system log. You might want to modify the crontab file to run a shell script nightly. The script searches the log for SYSERR messages and mails any messages that it finds to the postmaster.
Use the mailstats program to test mail types and determine the number of incoming messages and outgoing messages.