By default, the message transfers that occur between Hub Transport servers are encrypted and authenticated. You can only use Telnet on port 25 to submit messages to an internal Hub Transport server that has a Receive connector that is configured to allow anonymous access or Basic authentication to receive messages. Anonymous access is required for Internet-facing servers. When you send a message to a Receive connector that accepts Basic authentication, you must have a utility to convert the text strings that are used for the username and password into the Base64 format. Because the user name and password are easily discernable when Basic authentication is used, we don't recommend Basic authentication without encryption.

The Internet-facing SMTP messaging servers of many organizations are configured to validate the source IP address, the corresponding domain name system (DNS) domain name, and the reverse lookup IP address of any Internet host that tries to send a message to the server. If you connect a test computer to the Internet and try to send a test message to a remote messaging server by using Telnet on port 25, your message may be rejected. To satisfy security requirements that may be imposed by the remote messaging server, you can connect to the remote messaging server from your Edge Transport server. The accepted domains that are configured on the Edge Transport server have the appropriate DNS mail exchange (MX) records, address (A) records, and reverse lookup records that identify the Edge Transport server as a legitimate and traceable e-mail message source for those accepted domains on the Internet.

The procedures in this topic show how to use the Microsoft Telnet client that is included with Microsoft Windows Server 2003 and Windows XP. Earlier versions of Windows or third-party Telnet clients may require different syntax. The commands in the Windows Telnet client are not case-sensitive. The SMTP command verbs are capitalized for clarity.

You can't use the backspace key after you have connected to the destination SMTP server within the Telnet session. If you make a mistake as you type an SMTP command, you must press ENTER and then type the command again. Unrecognized SMTP commands or syntax errors result in an error message that resembles the following:

	Copy Code
500 5.3.3 Unrecognized command

To connect to a destination SMTP server by using Telnet on port 25, you must have the fully qualified domain name (FQDN) or the IP address of the SMTP server. The easiest way to find this information is to use the Nslookup command-line tool to find the MX record for the destination domain.

To use the Nslookup tool to find the IP address of an SMTP server

Note:

Firewall or Internet proxy restrictions that are imposed on your organization's internal network may prevent you from using the Nslookup tool to query public DNS servers on the Internet. MX records are not strictly required for internal message flow inside an Exchange organization. If you have to find the FQDN of any Hub Transport server or subscribed Edge Transport server in your organization, you can use the following command in the Exchange Management Shell: Get-ExchangeServer | where {$_.isHubTransportServer -eq $true -or $_.isEdgeServer -eq $true} | Format-List Fqdn,ServerRole For more information, see Get-ExchangeServer and Pipelining.

The procedure in this section shows how to use Telnet on port 25 to test SMTP communication. The procedure uses the values that are described in the following list:

• Destination SMTP server   mail1.fabrikam.com
  
  
• Source domain   contoso.com
  
  
• Sender's e-mail address   chris@contoso.com
  
  
• Recipient's e-mail address   kate@fabrikam.com
  
  
• Message subject   Test from Contoso
  
  
• Message body   This is a test message
  
  

Note:
You should always use a valid sender e-mail address so that any non-delivery report (NDR) messages that are generated by the destination SMTP server are delivered to the sender of the message.

To use Telnet on port 25 to test SMTP communication

This section provides details about the responses that may be provided to the commands that you entered in the previous example.

Note:
The 3-digit SMTP response codes that are defined in RFC 2821 are the same for all SMTP messaging servers. The text descriptions may differ slightly for some SMTP messaging servers. In this example, the destination computer is running Exchange Server 2007.

Open mail1.fabrikam.com 25

EHLO contoso.com

Successful Response   250 mail1.fabrikam.com Hello [<sourceIPaddress>]

Failure Response   501 5.5.4 Invalid domain name

Possible Reasons for Failure   There are invalid characters in the domain name. Alternatively, there are connection restrictions on the destination SMTP server.

Note:
EHLO is the Extended Simple Message Transfer Protocol (ESMTP) verb that is defined in RFC 2821. ESMTP servers can advertise their capabilities during the initial connection. These capabilities include their maximum accepted message size and their supported authentication methods. HELO is the older SMTP verb that is defined in RFC 821. Most SMTP messaging servers support ESMTP and EHLO.

MAIL FROM:chris@contoso.com

RCPT TO:kate@fabrikam.com NOTIFY=success,failure

For more information, see the following topics:

• Managing Connectors
  
  
• Managing Delivery Status Notifications