The documentation for CiviCRM has a fairly comprehensive guide to setting up your email. http://book.civicrm.org/user/current/advanced-configuration/email-system-configuration/
=== Postmaster ===Comcast has and others have their 'Postmaster' website websites with tools and info for help # http://postmaster.comcast.net/index.html# https://postmaster.aol.com/
Unfortunately, your hosting provider may be crippling your email service without even advertising the fact. https://warrenguy.me/blog/ipv6-digital-ocean-crippled