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Email tutorial?




I am trying once again to get an email server to run on my server. I NEED a qualified tutorial or some real assistance in getting it operational and secure. I am aware that there are MANY primers or docs on this. Problem is they like most are done for an individuals system and are not really designed for my system. So here is what I'm working with:
1. all IP addresses are DHCP regulated by Spectrum internet.
2. I do have a fully functioning Mediawiki website running on this server and it is just fine. Spectrum doesn't often change the IP addresses.
3. I have installed Dovecot and Postfix out of the box with no changes, for MTA and mail server
4. I have Thunderbird as my MUI.
5. All this is running on a system using Debian 9 (stable) with plenty of CPU and memory horsepower for the job.

I want to use this system to both send and receive email ONLY for this server. There is only one user account currently and that is mine. I need to be able to allow my Mediawiki system send replies to my membership and to receive queries and emails from that membership. Ther may be additional user accounts that need to be set up but for now, only mine. I have been sort of able to send a few test emails to my secondary testing account locally. Sending to an outside system such as my own Gmail or Yahoo simply does not work. I was getting an error message but I reinstalled everything again and am still  getting that message>
 
An error occurred while sending mail. The mail server responded:  
4.7.1 <jfoster81747@xxxxxxxxx>: Relay access denied.