To avoid delay in emails, specially for gmail. Just find which MX is working by issuing below commands:

# dig mx gmail.com

# telnet mx-IP/name 25

upon success, just see the name of first mx tried from /var/log/maillog

and add an entry in /etc/hosts file.

In my case, the entry is given below:
# vim /etc/hosts

74.125.25.26 aspmx.l.google.com
74.125.25.26 alt1.aspmx.l.google.com
74.125.25.26 alt2.aspmx.l.google.com

:save and exit

now open postfix main.cf and add smtp_host_lookup directive under inet_protocols = all

# vim /etc/postfix/main.cf

inet_protocols = all
smtp_host_lookup=native
save and exit

now restart postfix

# service postfix restart

by default postfix host lookup is done via DNS, native mode uses nsswitch.conf settings and will check /etc/hosts first and then DNS if entry not found in /etc/hosts

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