This article was previously published under Q297412
This article describes the Perform Reverse DNS Lookup for Incoming Messages option and how its function can be misinterpreted by Exchange administrators.
The Perform Reverse DNS Lookup for Incoming Messages option is located on the Default Virtual SMTP Server Properties dialog box: On the Delivery tab, click Advanced. Exchange administrators may misinterpret the function of this option: They may expect Exchange to reject e-mail messages that originate from unresolved domains.
Some messaging systems verify the existence of the e-mail domain of the sender before they accept a "Mail from: email@example.com" Simple Mail Transfer Protocol (SMTP) entry at the beginning of a new message delivery session. If the domain name cannot be resolved by means of Domain Name System (DNS), the session is disconnected and an error 501 is generated. This behavior is mainly used to prevent you from receiving spam (unsolicited e-mail messages). Microsoft Exchange Server 5.5 and later do not use this feature.
If you select the Perform Reverse DNS Lookup for Incoming Messages option, the SMTP service tries to verify that the client’s IP address matches the host and domain that is submitted by the client in the EHLO/HELO command. If the reverse DNS lookup is successful, the RECEIVED header remains intact. If the verification is unsuccessful, "unverified" appears after the IP address in the RECEIVED header of the message. If the reverse DNS lookup fails, "RDNS failed" appears in the RECEIVED header of the message.
Important Exchange does not change the Message Headers when the operation is successful.
If you enable the Perform Reverse DNS Lookup for Incoming Messages option, you may have some performance degradation issues because of misconfigured DNS records and/or intermittent connections to the Internet. Therefore, you may want to disable this option when the Internet mail delivery is slower than expected.
By default, Exchange Server 5.5 performs a reverse lookup operation on all connections. This default operation, however, can be disabled by using a DisableReverseResolve registry setting.
For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:
258745 XIMS: Internet Mail Service Displays SMTP Banner Slowly
198981 XIMS: SMTP Messages Not Being Delivered to Certain Domains
262571 XCON: Internet Mail Service Registry Entry for DisableReverseResolve Does Not Map to Default SMTP Virtual Server After You Upgrade to Exchange 2000