Sendmail did not send mail instantly but just queued it.
Testing sendmail by
System didn’t show any error but just queued the mail
Showing that the mail message has been put into the queue.
Queue message “host map: lookup (domain): deferred”
Can force server sent mail successfully. It means the DNS setting should be correct.
Check sendmail configuration files
DeliveryMode is not “deferred”
Both files have the same setting as another server which can send mail instantly.
Try to stop and restart sendmail