Thank you very much for the quick reply and suggestions.
I’ve gone through the Checkmk document, you suggested.
And I understand, it is fairly simple.
I have configured ~/etc/mail.rc to use our external smtp server.
From linux command line, tested sending mails using “mail” command which is working fine.
Only checkmk site is not sending notification mails.
Shall I share any more details which might help to understand the issue?