Before I begin, I want to explain what is DL and "Mail Enabled Security Groups".
"Mail Enabled Security Groups": This group can be used to enable permissions by group and also send emails to the members.
Distribution List: This group used only to send email and that’s it.
Now to my issue. I received complaint that two users weren’t receiving mails when it sent to a "Mail Enabled Security Group". All others are receiving mails with no issues.
Well it turn out to be if the user’s primary group is set to "Mail Enabled Security Group", user won’t receive mail sent to same group. So I made different group as primary group in Active Directory User Properties. That made two users happy that day. See the picture to speak 1000 words.