Updated DKIM how to (markdown)

DKIM-how-to.md

@@ -2,7 +2,23 @@
 This how to is created by the Dutch Internet Standards Platform (the organization behind [internet.nl](https://internet.nl)) and is meant to provide practical information and guidance on implementing DKIM.
 
 # What is DKIM?
-to-do. 
+DKIM stands for **D**omain**K**eys **I**dentified **M**ail and is described in RFC 6376](https://tools.ietf.org/html/rfc6376) with updates in [RFC 8301](https://tools.ietf.org/html/rfc8301) and {RFC 8463](https://tools.ietf.org/html/rfc8463). It is meant to provide the owner of a domain with the means to claim that a message has actually been send by the domain's e-mail server and should therefore be considered legitimate. It works by signing every individual e-mail message with a specific key (private key), so that the receiving party can use a corresponding key (public key) published in the sending domain's DNS record to validate the e-mail authenticity and to check whether the e-mail has not been tampered with. 
 
 # Why use DKIM?
-to-do.
+A common used technique used by spammers is to trick the receiving party into believing an e-mail is legitimate by using a forged sender address. This is also known as e-mail spoofing. DKIM has been designed to detect the use of spoofing. If an incoming e-mail does not have a DKIM signature or when it's DKIM signature does not validate, the receiving e-mail server should consider the e-mail to be SPAM.
+
+# Tips, tricks and notices for implementation
+* parked domain
+* minimum key length
+
+# Outbound e-mail traffic
+DNS record
+Signin in Postfix
+
+## Implementing DKIM in Postfix with SpamAssassin
+**Specifics for this setup**
+* Linux Debian 9.8 (Stretch) 
+* SpamAssassin version 3.4.2 (running on Perl version 5.28.1)
+* Postfix 3.4.5
+* BIND 9.10.3-P4-Debian
+* OpenDKIM v2.11.0