One of my friend told me most of their outgoing email are marked as Spam. Their email is xxx@abc.com. The official SMTP is delay.abc.com. But, my friend is not using that official SMTP address. Instead, they are using xyz.abc.com, which appear to be a sub domain.
Thanks to ‘Dig All Pointings of a Zone File‘. I managed to list out the list of pointing for abc.com. We discover xyz.abc.com is not listed as MX record.
Myth solved. Finally, they will add a new MX using xyz.abc.com to solve the Spam problem.
Spam problem can be solved. If you have such problem, let me know