After installing Zimbra as described at this guidance : installing Zimbra on CentOS 6 or install Zimbra on CentOS 7, by default we could sent email to outside/internet if have the internet connection and port 25 outgoing is not blocked by firewall or blocked by ISP connection. After that, usually try to receive email from internet/outside and never coming to Zimbra that has been installed. This problem occurs because some information below :
1. We should have the real domain that known on the internet. You can bought a domain from Godaddy or other provider
2. We should have at least one Public IP. You can rent a VPS from Digital Ocean or other provider and then install Zimbra in it
3. We should have dns management for configure A and MX Records. If you also bought hosting on your domain provider, usually you can manage dns on cPanel or something else. If you bought a domain only, you could using Free DNS management from Hurricane up to 50 domain and configure your domain nameserver to refer to Nameserver of Hurricane.
In this section, i have a domain imanudin.net that has been bought from Godaddy and have 1 Public IP 126.96.36.199 and dns management on cPanel (Hosting from Excellent Infotama Kreasindo Inc). This is what i do for a configuration to receive emails from internet/outside
# Configure A Records on DNS Management (cPanel)
Select Advanced DNS Zone Editor and add A records (see the following picture)
After adding A records, please check for dns propagation at this link : https://www.whatsmydns.net/ whether name zcs.imanudin.net (you can using another name) has been known in the internet or still propagate
# Configure MX Records on DNS Management (cPanel)
After configure A Records on cPanel, go to home and select MX Entry. If you have been saw existing MX records using 0 (zero) priority, change it to 50 or 20 (or you can remove it if not needed). Add MX records to refer to the name zcs.imanudin.net who has been configured formerly and give 10 priority.
After adding MX records, please check for dns propagation at this link : https://www.whatsmydns.net/ whether the new MX records for domain imanudin.net has been known in the internet or still propagate.
After above configuration has been configured, you should be able to receive emails from internet/outside.
Good luck and hopefully useful 😀