Sendmail servers can produce some wide range of problems that any Unix server can generate, most daily Sendmail issues fall into just a few categories which is related to mail connection, Sendmail relay configuration and SMTP auth issues.
1) Email not deliverable:
We can able to valid whether user email ID or local domain is able to deliver from the system.
#sendmail -bv usernameEmailID
#sendmail -bv root@localdomain
#sendmail -bv root@localdomain
2) Check the status of sendmail service. the mail server may go down if the server has a high workload.
#systemctl status sendmail
Start the sendmail if the service is down,
#systemctl start sendmail
3) Check the sendmail relay server details in the sendmail.cf configuration file.
#grep ^DS /etc/mail/sendmail.cf
The relay server should be resolve from the host, otherwise the mail request will not able to resolve by DNS and through Transient parse error -- message queued for future delivery error.
4) mqueue is got filled due to mail thread is not able to deliver or in the mail queue.
We used to face a var file system reached 100% utilization due to this problem.
i) check the current status of mail
#mailq
ii) Try to deliver the pending mail queue
#sendmail -v -q
iii) Stop the sendmail service
#systemctl stop sendmail
iv) move or delete the mqueue list
#mv /var/spool/mqueue/* /temporary_location
v) Start the sendmail service
#systemctl start sendmail
5) Validated the sendmail functions
i) send a testmail from the system
# echo "This is test email" | mailx -v -s "Test mail subject" -S smtp="smtpserver:port" "usermail_ID"
ii) Open a other terminal and monitor the mail thread
#tail -f /var/log/maillog
Sendmail Log location : /var/log/maillog
iii) Check sendmail connectivity from the system
#nc -vz sendmailsever 25
iv) check the connection of sendmail
#ps auxw| grep [a]ccepting
v) check the system is listening of sendmail
#netstat -antp| grep sendmail
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