You are unable to send or receive SMTP messages from certain Internet domains in Exchange 2000 Server, in Exchange Server 2003, and in Small Business Server 2003

Applies to: Microsoft Exchange Server 2003 Enterprise EditionMicrosoft Exchange Server 2003 Standard Edition


This article is a consolidation of the following previously available articles: 322358, 840468 and 924235

When you try to send or receive messages on a computer that is running Microsoft Exchange Server 2003, Microsoft Exchange 2000 Server, or MIcrosoft Windows Small Business Server 2003, you experience one of the following symptoms:
  • Exchange server does not accept Simple Mail Transfer Protocol (SMTP) messages from certain Internet domains.
  • Exchange server cannot deliver SMTP messages to certain Internet domains.
If you do a reverse Domain Name System (DNS) query, you may notice that the Exchange server that is sending the SMTP message cannot be resolved. If you take a Network Monitor trace, you may notice that there are NBT queries before the Exchange server disconnects.

The sender may receive the following non-delivery report (NDR) that contains the 5.5.0 error code. This code indicates a generic SMTP failure.

> Your message did not reach some or all of the intended recipients.
> Subject:
> Sent:5/24/01 6:41 PM
> The following recipient(s) could not be reached:
> on 5/24/01 6:41 PM
> Your mail system could not find a way to successfully communicate with the destination system. Please notify your administrator. <> #5.5.0
Note You may also receive the following error code: #5.5.4 Transaction failed.

Additionally, the Windows Event viewer on the Exchange server that is sending the message may contain an event 4000 or an event 4001 that is similar to the following:


Typically, this problem occurs if the destination SMTP server performs a reverse lookup and if one of the following conditions is true:
  • The IP address does not match the domain name that is used in the return address of the e-mail message.
  • A pointer (PTR) record does not exist or is not valid for the source SMTP server's IP address.
For example, consider the following scenario. The domain name that is used in the return address of the message is You send the message to a different domain. The destination SMTP server then performs a reverse lookup on the incoming message. If the PTR record for the domain does not exist or is incorrect, the destination SMTP server does not deliver the message.

Note If your Network adapter that is connected to the Internet is using a dynamic IP address, you may have to configure Exchange Server to route mail from the domain through an SMTP connector to a smart host.

Note You can configure the Exchange server to reject incoming connections by specifying a domain name on the SMTP virtual server. When this is done, reverse lookups are performed on all connection attempts. This setting is available under Connection Control on the Access tab when you right-click the SMTP virtual server and then click Properties.


To resolve this problem, follow these steps:
  1. Make sure that your public DNS records that are hosted on your DNS server are correct. On your DNS server, examine the following:
    • You must have an MX record for your domain that points to a valid Host (A) record. For example, the MX record for points to is a valid e-mail server.
    • Make sure that the Host (A) record points to a valid IP Address. For example, make sure that points to This is the correct public IP Address for your e-mail server.
  2. For every SMTP server or Exchange Server computer that sends outgoing Internet e-mail, make sure that there is a valid PTR record for the Public IP address of that sending SMTP server or Exchange Server computer. This may be a firewall, a router, or another device that used to publish your domain information to an IP address that is visible by Internet hosts.

    For example, your Exchange Server computer is behind a firewall with an internal IP of, and the firewall has an external IP of

    When the Exchange Server computer sends e-mail to source.comdomain through the firewall, the receiving mail server sees that the IP address is connecting for SMTP Communication. The receiving e-mail server performs a reverse DNS lookup against this IP address, not necessarily the MX record. The e-mail server must find a PTR for pointing to a valid host record in the domain.

More Information

Reverse Lookup causes additional overhead on servers, but is commonly used to help prevent environments from getting unsolicited e-mail messages (spam). You can configure your SMTP server to perform reverse lookups on incoming e-mail messages. This makes sure that the IP address and the fully qualified domain name (FQDN) of the sender's e-mail message server match the sender's domain name.

The mail servers of some Internet domains require that you create a valid PTR record that points the sending server's IP address to the local SMTP domain namespace. Sometimes these mail servers require that the PTR record match the actual FQDN of their SMTP Virtual Server on Exchange. Typically, this is the MX record. These Internet domains include,, Mindspring, Earthlink, and To send mail to these domains, create a valid PTR or a reverse lookup record on your company’s external or Internet DNS server.

If you telnet to the destination server's SMTP port and then attempt an SMTP conversation, the conversation will look similar to the following:
C:\telnet 25 
220 ESMTP Postfix
250 Ok
554 <>: Sender address rejected: Domain not found
Additionally, if you telnet to port 25 on one of the AOL MX records, the following appears: ESMTP mail_relay_in-ya6.3; Fri, 20 Jan 2006 22:15:42 -05 00 
220-America Online (AOL) and its affiliated companies do not
220- authorize the use of its proprietary computers and computer
220- networks to accept, transmit, or distribute unsolicited bulk
220- e-mail sent from the internet. Effective immediately: AOL
220- may no longer accept connections from IP addresses which
220 have no reverse-DNS (PTR record) assigned.


This behavior is by design.


For more information about reverse DNS lookup for incoming messages, click the following article number to view the article in the Microsoft Knowledge Base:

297412 The "Perform Reverse DNS Lookup for Incoming Messages" option is for host name resolution

For more information about PTR records, click the following article numbers to view the articles in the Microsoft Knowledge Base:

166753 Microsoft DNS server reverse lookup error adding host record

322856 How to configure DNS to use with Exchange Server

For more information about DNS and about reverse lookup zones, visit the following Microsoft Web site:
For more information about configuring an SMTP connector in Exchange 2000 and Exchange Server 2003, click the following article number to view the article in the Microsoft Knowledge Base:

265293 How to configure the SMTP connector in Exchange

For more information about how to use port 25 to test SMTP communication, click the following article number to view the article in the Microsoft Knowledge Base:

153119 Telnet to port 25 to test SMTP communication

For more information about similar problems in Microsoft Exchange Server 5.5, click the following article number to view the article in the Microsoft Knowledge Base:

198981 SMTP messages not being delivered to certain domains