Using Telnet to test email (SMTP)

Introduction

This document describes how to use telnet to perform a basic SMTP test from your local client through a configured listener on your Email Security Appliance (ESA) or Security Management Appliance (SMA).

Using Telnet to test email (SMTP)

If you are troubleshooting email connectivity and email reciept issues, it may be necessary to test basic SMTP communication for the problem that you are experiencing. For example, if you have problems sending email messages over SMTP to your ESA, you can test the SMTP connectivity by using Telnet on your local client or desktop, and connect to port 25 on your ESA. By default, SMTP listens on port 25.

Note: Confirm what port your listener is configured on for your ESA from Network > Listeners on the GUI, or listenerconfig on the CLI.

Example:

$ telnet 172.16.6.165 25
Trying 172.16.6.165...
Connected to my_esa.
Escape character is '^]'.
220 my_esa.local ESMTP
helo
250 my_esa.local
mail from: <test@test.com>
250 sender <test@test.com> ok
rcpt to: <user@other.com> 
250 recipient <user@other.com> ok
data
354 go ahead
subject: TESTING SMTP
This is line one.
This is line two.
. 
250 ok: Message 214 accepted
quit
221 my_esa.local
Connection closed by foreign host.

Note: Any output code other than 2xx is generally an error

Commonly Seen SMTP Error Codes

4xx Codes:
    421 #4.4.5 Too many TLS sessions at this time
    421 #4.4.5 Too many connections from your host
    421 #4.4.5 Too many connections to this host
    421 #4.4.5 Too many connections to this listener
    421 #4.x.2 Too many messages for this session
    421 <hostname> Service not available, closing transaction channel
    421 Exceeded allowable connection time
    421 Exceeded bad SMTP command limit, disconnecting
    421 The evaluation license has expired
    451 #4.3.0 Server Error
    452 #4.3.1 queue full
    452 #4.3.1 server resources low - try again later
    452 #4.3.1 temporary system error (12)
    452 #4.5.3 Too many recipients
    454 TLS not available due to a temporary reason

5xx Codes:
    500 #5.5.1 command not recognized
    500 Line too long
    501 #5.0.0 EHLO requires domain address
    501 #5.5.2 syntax error XXX
    501 #5.5.4 Invalid arguments to AUTH command
    501 Unknown command XXX
    501 Unknown option XXX
    501 Unknown value XXX
    503 #5.3.3 AUTH not available
    503 #5.5.0 AUTH not permitted during a mail transaction
    503 #5.5.0 Already authenticated
    503 #5.5.1 MAIL first
    503 #5.5.1 RCPT first
    503 Bad sequence of commandsDATA within mailmerge transaction
    503 Bad sequence of commandsXPRT within plain transaction
    503 Bad sequence of commandsnow receiving parts
    503 Not in a mailmerge transaction
    504 #5.5.1 AUTH mechanism XXX is not available
    504 Command parameter XXX unrecognized
    504 Invalid XDFN syntax
    504 Invalid part number
    504 Invalid part number XXX
    504 No variable value specified
    504 Other parts still missing
    504 Reserved variable name
    504 Syntax error in *parts syntax
    504 XDFN command must not contain NULL characters
    530 #5.7.0 Must issue a STARTTLS command first
    530 #5.7.0 This sender must issue a STARTTLS command first
    530 Authentication required
    538 #5.7.11 Encryption required
    552 #5.3.4 message header size exceeds limit
    552 #5.3.4 message size exceeds limit
    552 size limit exceeded
    554 #5.3.0 Server Error
    554 Too many hops
    554 message body contains illegal bare CR/LF characters.

Related Information

• Cisco Email Security Appliance - End-User Guides
• Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry 
• Technical Support & Documentation - Cisco Systems