Why rsyslog does not process log messages from remote hosts?

Issue

• The rsyslogd daemon is set up to receive log messages from remote clients and listening on the UDP port, which has been verified open on firewalls.
• Syslog messages from clients are being received on the network interface, but rsyslog does not process those.

• Messages, similar to the following, are visible in /var/log/messages, matching IP addresses of the clients:

  Oct  4 08:15:43 host.example.com kernel: [26988714.572081] IPv4: martian source <server_address> from <client_address>, on dev <interface>