Monthly Archives: August 2008

Telnet: the best email diagnostic tool

Oftentimes it is very helpful to remove your mail server from the
equation to see if there is an underlying network / reputation problem
blocking mail flow. Here’s how you can manually send a test message
using the telnet command built into every operating system. You’ll need
to determine the name or address of your recipeints mail server. You
can do this by looking up their MX record at In
my example the MX record is Your commands are in bold
below and the responses I got are in italic. Be careful, on Windows if
you typo you cannot backspace and correct yourself. Just hit enter and
retype the entire command.

telnet 25
Connected to (
Escape character is ‘^]’.

mail from: <>
250 sender <> ok
rcpt to: <>
250 recipient <> ok
354 go ahead
Subject: Test Message
This is a test message.

250 ok:  Message 156715331 accepted
Connection closed by foreign host.

you can see that the sender, recipient and message were accepted by the
250 responses from the recieving mail server. If there are problems you
will see them reflected with 4xx or 5xx responses that can be very
helpful for figuring out the problem.

Product Development Engineer