==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.7 MYSQL Edition
- Linux/BSD distribution name and version: Ubuntu 16.04 Server
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have a very weird situation going on that I cannot seem to find resolution to when I search online or in these forums.  I host several domains on my mail server for customers and mail is coming in, but only from select domains. For example, mail is being delivered from domains such as Airbnb.com, gmail.com, mailchimp.com, yahoo.com, and some other places.  However, messages are not being received from most other domains we have messages sent from, including facebook.com and other popular domains.  Mail from many private ISP domains are not being delivered, either.  The sender never receives an error message, and when I look in the mail log, there is never any record of these messages reaching the server.  The only record of messages receiving the server are the ones from the domains that are being delivered from that I mentioned above and some other domains, but not many.  I am at a roadblock now on this and need help.  Thanks!