Sendmail did not send mail instantly but just queued it.

Testing sendmail by

#mail test@host.com

System didn’t show any error but just queued the mail

#mailq

Showing that the mail message has been put into the queue.

Queue message  “host map: lookup (domain): deferred”

#/sendmail -q

Can force server sent mail successfully. It means the DNS setting should be correct.

Check sendmail configuration files

#/etc/mail/sendmail.cf

DeliveryMode is not  “deferred”

O DeliveryMode=background

#/etc/hosts

#/etc/resolv.cf

#/etc/nsswitch.conf

Both files have the same setting as another server which can send mail instantly.

Try to stop and restart sendmail

#/etc/init.d/sendmail stop

#/etc/init.d/sendmail start

Problem solved.