zabbix 3.2 -- Delaying sending alert e-mails?

Issue:

Alerts are delayed from Zabbix. it was arround 30 mins late arriving to the Admin's Inbox.
While Checking in the zabbix found that the E-mail is being processed and it was stuck their
for 29 mins. Then try sending the e-mail from zabbix server from command prompt. it was stucking
aswell. Then installe postfix (MTA) and configure it and started. Then the mail started to flow.
That get down the delay interval to 13 min.

Install postfix
#yum install postfix -y

enable the service
#systemctl enable postfix.service

Start the service
#/bin/systemctl start postfix.service

Check the Status of the service
#/bin/systemctl status postfix.service

In the Zabbix Dashboard go to Administration --> Media types --> Email
then put the Correct SMTP Server Name, SMTP server port and Sender E-mail id.
then Click on Update.