Questions about proper e-mail server configuration, with those settings being properly configured to ensure e-mail delivery, is a very difficult question and is, largely, being driven by YAHOO! and OUTLOOK.COM (which now hosts all of outlook.com, private domains which are hosted in the outlook cloud, and all of the HOTMAIL.com e-mail services).
The changes driven by YAHOO! and OUTLOOK.COM, along with GMAIL, COMCAST, and several other large providers (I am deliberately not including AOL in here because they are totally fubared at this point), have completely changed the game and mean that e-mail is no longer an add-on for hosted services, but something which must be monitored and continuously fine-tuned to keep it working.
Remember, if you host more than one domain on your mail server, you must implement the procedures outlined in the document for EVERY ONE of the hosted domains.
Bruce Barnes
ChicagoNetTech Inc
brucecnt@comcast.net
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Website: https://www.ChicagoNetTech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting