Cisco Cisco Email Security Appliance C370 User Guide
Chapter 5 Configuring the Gateway to Receive Email
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Cisco IronPort AsyncOS 7.1 for Email Configuration Guide
OL-22158-02
Note
The system acquires and verifies the validity of the remote host’s IP address by
performing a double DNS lookup. This consists of a reverse DNS (PTR) lookup
on the IP address of the connecting host, followed by a forward DNS (A) lookup
on the results of the PTR lookup. The system then checks that the results of the A
lookup match the results of the PTR lookup. If the results do not match, or if an
A record does not exist, the system only uses the IP address to match entries in
the HAT.
performing a double DNS lookup. This consists of a reverse DNS (PTR) lookup
on the IP address of the connecting host, followed by a forward DNS (A) lookup
on the results of the PTR lookup. The system then checks that the results of the A
lookup match the results of the PTR lookup. If the results do not match, or if an
A record does not exist, the system only uses the IP address to match entries in
the HAT.