PuTTY closes when opening SMTP over Telnet on certain server

puttysmtptelnet

I have PuTTY installed on Vista. When I set it to Telnet and put in a host name of SMTP2Go.com and port of 25 the window opens for a second not showing any text then closes. I also tried ports 2525, 587, and 8025, which SMTP2Go says also work with the same result. I use Telnet from the command prompt (which I had to enable from the control panel) to open smtp2go.com with port 25 and it works. I also can use both PuTTY and the other Telnet to connect to outgoing.verizon.net. I am using Frontier for my ISP and it says on their site that port 25 is blocked, and other people have confirmed this, so I find it strange that I can access SMTP2Go on port 25 using Microsoft Telnet. Could SMTP2Go determine what client I'm using and block certain ones?

Also in Microsoft Telnet there aren't line breaks between my commands and the server's response with Verizon, but with PuTTY there are.

Best Answer

Just select "Raw" in the "Connection type" field and it will work OK.

From PuTTY's documentation:

A lot of Internet protocols are composed of commands and responses in plain text. For example, SMTP (the protocol used to transfer e-mail), NNTP (the protocol used to transfer Usenet news), and HTTP (the protocol used to serve Web pages) all consist of commands in readable plain text.

Sometimes it can be useful to connect directly to one of these services and speak the protocol ‘by hand’, by typing protocol commands and watching the responses. On Unix machines, you can do this using the system's telnet command to connect to the right port number. For example, telnet mailserver.example.com 25 might enable you to talk directly to the SMTP service running on a mail server.

Although the Unix telnet program provides this functionality, the protocol being used is not really Telnet. Really there is no actual protocol at all; the bytes sent down the connection are exactly the ones you type, and the bytes shown on the screen are exactly the ones sent by the server. Unix telnet will attempt to detect or guess whether the service it is talking to is a real Telnet service or not; PuTTY prefers to be told for certain.

In order to make a debugging connection to a service of this type, you simply select the fourth protocol name, ‘Raw’, from the ‘Protocol’ buttons in the ‘Session’ configuration panel.

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Windows – Cannot connect to telnet server

Have you tried disabling the auto-tuning diagnostic in Windows 7?

netsh int tcp set global autotuninglevel=disabled

Microsoft Support KB article:

When the receive window autotuning feature is enabled, older routers, older firewalls, and older operating systems that are incompatible with the receive window autotuning feature may sometimes cause slow data transfer or a loss of connectivity.

The default auto-tuning level is "normal", and the possible settings for the above command are:

disabled: uses a fixed value for the tcp receive window. Limits it to 64KB (limited at 65535).
highlyrestricted: allows the receive window to grow beyond its default value, very conservatively
restricted: somewhat restricted growth of the tcp receive window beyond its default value
normal: default value, allows the receive window to grow to accommodate most conditions
experimental: allows the receive window to grow to accommodate extreme scenarios (not recommended, it can degrade performance in common scenarios, only intended for research purposes. It enables RWIN values of over 16 MB)

Windows – Testing SMTP via Telnet, “help” returns “500 unrecognized command”

Your mail server is telling you it has no help command. Thats what its designed to do

I've based my answer off the absolutely brilliant CC licensed guide off flurdy.com. His version is clearer - so check that out (he has a whole section for troubleshooting), and the whole guide is worth reading and understanding - he covers EVERYTHING you will need. I've made a few changes, such as assuming you're already telnetted in, and I've commented out where he's mentioned the expected output for more clarity, since I don't have the benefit of coloured text. The rest of this answer is primarily for search-ability and redundancy. Use the non commented lines one line at a time, don't copy and paste them since this is basically telling the mail server what it wants to hear, as if you were a mail client.

# Open the hand shake with ehlo and the server name you are connecting from... 
# This time it has to be the name of your server 
EHLO mail.example.org 
# The mail server will then dump out some details about its capabilities, e.g. 
#>250-mail.flurdy.net 
#>250-PIPELINING 
#>.... 
#>.... #
 then say who is the sender of this email, which is a local user 
MAIL FROM: <xandros@example.org> 
> 250 Ok 
# then say who the mail is for which is an external address e.g. gmail etc. 
RCPT TO: <you@example.com> 
> 250 Ok 
# then enter the keyword data 
data 
 > 354 End data with <CR><LF>.<CR><LF></LF></CR></LF></CR> 
# enter message body and end with a line with only a full stop. 
 blah blah blah 
 more blah 
 . 
 #> 250 Ok; queued as QWKJDKASAS 
 # end the connection with 
 quit 
 > 221 BYE

once thats done, check if you've received the message. If not, check your logs for error messages - this should be /var/log/mail.log.

Best Answer

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Windows – Testing SMTP via Telnet, “help” returns “500 unrecognized command”

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