Applies to: Exchange Server 2013
Topic Last Modified: 2012-09-10
This topic explains how to use Telnet to test Simple Mail Transfer Protocol (SMTP) communication between messaging servers. By default, SMTP listens on port 25. If you use Telnet on port 25, you can enter the SMTP commands that are used to connect to an SMTP server and send a message exactly as if your Telnet session was an SMTP messaging server. You can see the success or failure of each step in the connection and message submission process.
Here are the scenarios where you may want to use Telnet to test SMTP communication to or from the transport servers that exist in your Microsoft Exchange organization:
- Connect to your organization's Internet-facing Exchange server
from a host that is located outside your perimeter network and send
a test message.
- Connect to a remote messaging server from your organization's
Internet-facing Exchange server and send a test message.
The procedure in this topic shows you how to use Telnet Client, which is a component that is included with Microsoft Windows. Third-party Telnet clients may require a syntax that is different from that of the Windows Telnet component.
What do you need to know before you begin?
- Estimated time to complete: 30 minutes
- Exchange permissions don't apply to the procedures in this
topic. These procedures are performed in the operating system of
the Exchange Server or a client computer.
- The procedures in this topic are best used to connect to and
from Internet-facing servers that allow anonymous connections.
Message transmission between internal Exchange servers is encrypted
and authenticated. To use Telnet to connect to the Hub Transport
service on a Mailbox server, you'll need to create a Receive
connector that's configured to allow anonymous access or Basic
authentication to receive messages. If the connector allows Basic
authentication, you need 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 discernible when
Basic authentication is used, we don't recommend Basic
authentication without encryption.
- If you connect to a remote messaging server, consider
performing the procedures in this topic on your Internet-facing
Exchange server. This will help to avoid rejection of the test
message by remote messaging servers that 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.
- For information about keyboard shortcuts that may apply to the
procedures in this topic, see Keyboard Shortcuts in
the Exchange Admin Center.
How do you do this?
Step 1: Install the Telnet Client in Windows
By default, the Telnet Client isn't installed in most client or server versions of the Microsoft Windows operating systems. To install it, see Install Telnet Client.
Step 2: Use Nslookup to find the FQDN or IP address in the MX record of the remote SMTP server
To connect to a destination SMTP server by using Telnet on port 25, you must use the fully qualified domain name (FQDN) or the IP address of the SMTP server. If the FQDN or IP address is unknown, the easiest way to find this information is to use the Nslookup command-line tool to find the MX record for the destination domain.
- At a command prompt, type nslookup, and then press
ENTER. This command opens the Nslookup session.
- Type set type=mx and then press ENTER.
- Type set timeout=20 and then press ENTER. By default,
Windows DNS servers have a 15-second recursive DNS query time-out
- Type the name of the domain for which you want to find the MX
record. For example, to find the MX record for the fabrikam.com
domain, type fabrikam.com., and then press ENTER.
Note: The trailing period ( . ) indicates a FQDN. The use of the trailing period prevents any default DNS suffixes that are configured for your network from being unintentionally added to the domain name.
fabrikam.com mx preference=10, mail exchanger = mail1.fabrikam.com fabrikam.com mx preference=20, mail exchanger = mail2.fabrikam.com mail1.fabrikam.com internet address = 192.168.1.10 mail2 fabrikam.com internet address = 192.168.1.20
- When you're ready to end the Nslookup session, type
exit, and then press ENTER.
|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.|
Step 3: Use Telnet on Port 25 to test SMTP communication
In this example, the following values are used:
- Destination SMTP
- Source domain contoso.com
- Sender's e-mail
- Recipient's e-mail
- Message subject Test from Contoso
- Message body This is a test message
- At a command prompt, type telnet, and then press ENTER.
This command opens the Telnet session.
- Type set localecho and then press ENTER. This optional
command lets you view the characters as you type them. This setting
may be required for some SMTP servers.
- Type set logfile <filename>. This optional
command enables logging of the Telnet session to the specified log
file. If you only specify a file name, the location of the log file
is the current working directory. If you specify a path and a file
name, the path must be local to the computer. Both the path and the
file name that you specify must be entered in the Microsoft DOS 8.3
format. The path that you specify must already exist. If you
specify a log file that doesn't exist, it will be created for
- Type open mail1.fabrikam.com 25 and then press
- Type EHLO contoso.com and then press ENTER.
- Type MAIL FROM:firstname.lastname@example.org and then press
- Type RCPT TO:email@example.com NOTIFY=success,failure
and then press ENTER. The optional NOTIFY command defines the
particular delivery status notification (DSN) messages that the
destination SMTP server must provide to the sender. DSN messages
are defined in RFC 1891. In this case, you're requesting a DSN
message for successful or failed message delivery.
- Type DATA and then press ENTER. You will receive a
response that resembles the following:
354 Start mail input; end with <CLRF>.<CLRF>
- Type Subject: Test from Contoso and then press
- Press ENTER. RFC 2822 requires a blank line between the
Subject:header field and the message body.
- Type This is a test message and then press ENTER.
- Press ENTER, type a period ( . ) and then press ENTER.
You will receive a response that resembles the following:
250 2.6.0 <GUID> Queued mail for delivery
- To disconnect from the destination SMTP server, type
QUIT and then press ENTER. You will receive a response that
resembles the following:
221 2.0.0 Service closing transmission channel
- To close the Telnet session, type quit and then press
Step 4: Evaluate the Results of the Telnet Session
This section provides information about responses that may be provided to the following commands, which were used in the previous example:
- Open mail1.fabrikam.com 25
- EHLO contoso.com
- MAIL FROM:firstname.lastname@example.org
- RCPT TO:email@example.com NOTIFY=success,failure
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.
Open mail1.fabrikam.com 25
mail1.fabrikam.com Microsoft ESMTP MAIL Service ready at
mail1.fabrikam.com...Could not open connection to the host, on port
25: Connect failed
Possible Reasons for Failure
- The destination SMTP service is unavailable.
- There are restrictions on the destination firewall.
- There are restrictions on the source firewall.
- An incorrect FQDN or IP address for the destination SMTP server
- An incorrect port number was specified.
mail1.fabrikam.com Hello [<sourceIPaddress>]
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.
|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.|
2.1.0 Sender OK
5.1.7 Invalid address
Possible Reasons for Failure There is a syntax error in the sender's e-mail address.
5.7.1 Client was not authenticated
Possible Reasons for Failure The destination server does not accept anonymous message submissions. You receive this error if you try to use Telnet to submit a message directly to a Hub Transport server.
RCPT TO:firstname.lastname@example.org NOTIFY=success,failure
2.1.5 Recipient OK
5.1.1 User unknown
Possible Reasons for Failure The specified recipient does not exist in the organization.