VCSA 6.7 U1 unable to send alert mails to different domain

I have recently migrated from Windows based vCenter server 6.0 U3 to VCSA 6.7 U1. With windows based vCenter, I was able to receive alerts on my mail box. Post migration alerts were not coming. 

After analyzing the logs I understand that, my VCSA 6.7 U1 is in XYZ.domain.com which is not exposed. And I was trying to send mail to ABC.domain.com which is exposed in internet. So VCSA 6.7 U1 was unable to resolve the DNS of ABC.domain.com. Below are the changes which I have done to send mail to other domain from VCSA 6.7 U1.

Take a backup of "sendmail.cf". Create new file "service.switch" and perform entry as given below.

[root@xxxxxxx01 ~]# cat /etc/mail/service.switch
hosts files

Later search "O ResolverOptions" in sendmail.cf. Default option will be "#O ResolverOptions=+AAONLY".

un-comment this option and update as below.

O ResolverOptions=-DNSRCH

Restart the sendmail service "service sendmail restart" and wait for 2 mins.

Now VCSA 6.7 will stop doing DNS resolution of ABC.domain.com and you will start getting alert mails on mail@ABC.domain.com.