Add missing content to DevOps node (#2064)

* Add missing content to Devops node

Added missing content to 100-white-grey-listing.md node

* Update content/roadmaps/102-devops/content/103-networking-protocols/103-emails/100-white-grey-listing.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/102-devops/content/103-networking-protocols/103-emails/100-white-grey-listing.md

@@ -1 +1,9 @@
-# White grey listing
+# White Listing vs Grey Listing
+
+White listing is a process of adding an email to an approved sender list, so emails from that sender are never moved to the spam folder. This tells an email server to move messages to the inbox directly.
+
+`Greylisting` is a method of protecting e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and the email will be accepted if sufficient time has elapsed.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.cblohm.com/blog/education-marketing-trends/what-is-email-whitelisting/'>Basic Introduction to whitelisting</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://en.wikipedia.org/wiki/Greylisting_(email)'>Detailed Introduction to greylisting</BadgeLink>