Test your mail server

Basic Sending of Mail

As a first basic test, try to send mail using sendmail. You'll want to log in to the server using ssh. Then on the server, create the file ~/samplemail:

$ sendmail -v -F fromname -f from@example.com to@example.com
From: from@example.com
To: to@example.com
Subject: Alpha Bravo
Date: Wed, 18 Sep 2024 16:26:35 -0700
MIME-Version: 1.0 
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

Charlie Delta Echo Foxtrot

Replace fromname with the name of the sender (your name), from@example.com with the sender's address, and to@example.com with the email you want to send to.

Press ctrl+d to end the letter.

You should see the following output:

<<< 220 example.com ESMTP OpenSMTPD
>>> EHLO localhost
<<< 250-example.com Hello localhost [local], pleased to meet you
<<< 250-8BITMIME
<<< 250-ENHANCEDSTATUSCODES
<<< 250-SIZE 36700160
<<< 250-DSN
<<< 250 HELP
>>> MAIL FROM:<from@example.com>  
<<< 250 2.0.0 Ok
>>> RCPT TO:<to@example.com> 
<<< 250 2.1.5 Destination address valid: Recipient ok
>>> DATA
<<< 354 Enter mail, end with "." on a line by itself
>>> .
<<< 250 2.0.0 379ec228 Message accepted for delivery
>>> QUIT
<<< 221 2.0.0 Bye

Sending with your mail client

You should also send email from a known, working email address to your new mail server. Afterwards, configure your e-mail client to see if you were able to receive the letter.

Sending with netcat

You can test to see if you can send a letter to your server using netcat:

$ nc example.com 25 
220 example.com ESMTP OpenSMTPD

Next, we type HELO followed by our sending domain:

HELO example.com
250 example.com Hello example.com [38.81.163.143], pleased to meet you

Afterwards, we type our sending mail address:

MAIL FROM: <from@example.com>
250 2.0.0 Ok

And the destination mail address:

RCPT TO: <to@example.com>
250 2.1.5 Destination address valid: Recipient ok

Then we type DATA followed by our email:

DATA
354 Enter mail, end with "." on a line by itself
From: from@example.com
To: to@example.com
Subject: Alpha Bravo
Date: Wed, 18 Sep 2024 16:26:35 -0700
MIME-Version: 1.0 
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

Charlie Delta Echo Foxtrot

We then type . to end the email, then QUIT:

.
250 2.0.0 e57f9a36 Message accepted for delivery
QUIT
221 2.0.0 Bye

Here's the complete process:

$ nc example.com 25 
220 example.com ESMTP OpenSMTPD
HELO example.com
250 example.com Hello example.com [38.81.163.143], pleased to meet you
MAIL FROM: <from@example.com>
250 2.0.0 Ok
RCPT TO: <to@example.com>
250 2.1.5 Destination address valid: Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
From: from@example.com
To: to@example.com
Subject: Alpha Bravo
Date: Wed, 18 Sep 2024 16:26:35 -0700
MIME-Version: 1.0 
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

Charlie Delta Echo Foxtrot
.
250 2.0.0 e57f9a36 Message accepted for delivery
QUIT
221 2.0.0 Bye

Open Mail Relay

Make sure your mail server is not an open relay!

$ nc example.com 25 
220 example.com ESMTP OpenSMTPD
HELO fakedomain.com
250 example.com Hello fakedomain.com [38.81.163.143], pleased to meet you
MAIL FROM: <criminal@fakedomain.com>
250 2.0.0 Ok
RCPT TO: <victim@otherplace.com>
550 Invalid recipient: <victim@otherplace.com>

In this example, you are pretending to send email from criminal@fakedomain.com to victim@otherplace.com using example.com as a relay.

You should get Invalid recipient or some similar rejection message. If you do not, example.com is likely running an open mail relay.

Here's how it appears when the mail server is improperly configured:

$ nc example.com 25 
220 example.com ESMTP OpenSMTPD
HELO fakedomain.com
250 example.com Hello fakedomain.com [38.81.163.143], pleased to meet you
MAIL FROM: <criminal@fakedomain.com>
250 2.0.0 Ok
RCPT TO: <victim@otherplace.com>
250 2.1.5 Destination address valid: Recipient ok

Notice this time it says Recipient ok even though it comes from a spoofed sender to an external domain. This will allow anyone to use your server to spam another mail server with forged addresses. If you don't fix this, you will get blacklisted for spam!

Testing Deliverability

URL	Description
https://dkimvalidator.com/	Send a test email and see if DKIM/SPF validation passes
http://multirbl.valli.org	Send a test email and look for any red flags
https://dmarc.org/resources/deployment-tools/
https://toolbox.googleapps.com/apps/checkmx
https://postmaster.google.com/u/0/managedomains
https://www.mail-tester.com
https://www.spamscore.net/
https://glockapps.com/
https://www.senderscore.org/
https://www.gmass.co/inbox
autoreply [snail] dmarctest [period] org
check-auth [snail] verifier [period] port25 [period] com
https://campaigncleaner.com/tools/mail-tester/

How to Fix

You will want to check your smtpd.conf ruleset to make sure you never allow any domain to send to any domain. You should only allow local hosts to send to external domains, and for any host to send to your domains.