- Inbound email messages cannot be delivered to the mail account on domain example.com.
- There are no bounce messages
Domain example.com does not have MX record:
# nslookup mx example.com
*** UnKnown can't find mx: No response from server
- Or MX record points to the mail server with incorrect IP address
Not properly configured MX records for the domain. Most likely they point to some foreign server.
Configure MX records on the server where main DNS zone for the server is hosted.