Add dkimvalidator.com under mail section.

README.md

@@ -281,6 +281,7 @@ Before add pull request please see **[this](https://github.com/trimstray/the-boo
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://mxtoolbox.com/SuperTool.aspx"><b>MX Toolbox</b></a> - all of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://www.blacklistalert.org/"><b>blacklistalert</b></a> - checks to see if your domain is on a Real Time Spam Blacklist.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="http://multirbl.valli.org/"><b>MultiRBL</b></a> - complete IP check for sending Mailservers.<br>
+&nbsp;&nbsp;:small_orange_diamond: <a href="http://dkimvalidator.com/"><b>DKIM SPF & Spam Assassin Validator</b></a> - Checks mail authentication and scores messages with Spam Assassin<br>
 </p>
 
 ##### :black_small_square: Net-tools