How to test SMTP servers, SMTP authentication and StartTLS using the command line?
When investigating SMTP authentication issues, particular over TLS encrypted SMTP connections, it’s always handy if you are able to test the SMTP authentication and StartTLS connection. Preferably from your command line. This post shows you how to test SMTP servers, verify SMTP authentication and StartTLS encrypted connections from the Linux and Windows command line.
SMTP Authentication is the mechanism by which the clients of an ISP identify themselves to the mail server through which they intend to send email.
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol whereby an SMTP client may log in using an authentication mechanism chosen among those supported by the SMTP server.
What is Transport Layer Security (TLS)?
Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols which are designed to provide communication security over the Internet. They use X.509 certificates and hence asymmetric cryptography to assure the counterparty with whom they are communicating, and to exchange a symmetric key.
Test TLS connections and SMTP AUTH from the Linux and Windows command line
Most SMTP and mail sending problems come from the fact that either the username and password log-in combination is incorrect, the mail server doesn’t support StartTLS, or the authentication mechanism used is wrong.
Let’s address, test and verify them all.
Being able to verify (Start)TLS encrypted connections with OpenSSL, and SMTP AUTH options, is ideal for when you’re having problems with email forms that send email using authenticated SMTP, over an TLS encrypted connection (fom a website).
To verify SMTP authentication over TLS, you need the OpenSSL client:
sudo apt-get install openssl sudo yum install openssl
Create SMTP AUTH log-in information with Perl
Before you can test the SMTP AUTH PLAIN authentication over TLS, you need to create log-in information. The log-in information is your usename (email address) and password, and a special character
\0. Normally this is an email address and its password.
To create the combination – which has to be base64 encoded – you can use Perl:
perl -MMIME::Base64 -e 'print encode_base64("\000username\@example.com\000password")'
You must not forget to escape the
@ char with a slash (
\), otherwise it’ll be interpreted as an array. The base64 encoded string will be something like:
SMTP AUTH log-in information with Bash
You don’t necessarily need Perl to generate a log-in hash, you can use Bash too:
echo -ne '\firstname.lastname@example.org\0password' | base64
Connect to an SMTP server with openssl in bash
In bash you now can use the
openssl command, as explained below, to set up a TLS encrypted connection with your SMTP server:
openssl s_client -connect smtp.example.com:25 -starttls smtp
This gives a lot of verbose output, don’t worry :)
When the connection is made, you’ll notice an SMTP 250 code:
This means you can start your SMTP transaction. Use
EHLO to let the SMTP server print out the supported verbs:
EHLO there 250-smtp.example.com 250-PIPELINING 250-SIZE 52428800 250-ETRN 250-AUTH PLAIN LOGIN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN
Here you notice
AUTH PLAIN LOGIN as a log-in method. The SMTP mail server supports the authentication mechanism you want. Your complete username and password log-in information is wrapped in the base64 encoded string. Use that to authenticate:
AUTH PLAIN AHVzZXJuYW1lQGV4YW1wbGUuY29tAG15X3Bhc3N3b3Jk
If all goes well, the SMTP server reports a successful authentication:
235 2.7.0 Authentication successful
Because the username and password combination is base64 encoded, and is sent in plain text, you need StartTLS/TLS encryption to secure your SMTP connection.
How to install OpenSSL and Perl on Windows
In order to accomplish all of the above on Windows Server or Windows 8.1 or 10, you need to download and install the OpenSSL client and Perl (I use Strawberry Perl):
- Win32 OpenSSL Installation Project (choose the right flavor)
- Strawberry Perl Releases (I use the ZIP edition which doesn’t require an installation)
- Install OpenSSL to
c:\OpenSSL-Win64, depending on the bitness
strawberry-perl-220.127.116.11-64bit.zipand copy the folder to c:\Perl for example
Configure your Windows OpenSSL environment
Now configure your OpenSSL environment in Windows to prevent
- at the cmd.exe command line, type
set OPENSSL_CONF=c:\OpenSSL-Win64\bin\openssl.cfg. This will prevent an error message:
WARNING: can't open config file: /usr/local/ssl/openssl.cnf
Create SMTP AUTH log-in information and connecto to an SMTP server with Perl/openssl in Windows
Use the following Perl command to generate the base64 encoded log-in string. Notice the quotation marks:
perl.exe -MMIME::Base64 -e "print encode_base64(\"\000username\@example.com\000password\")
In this example the output is
Connect to your SMTP server with
openssl.exe, and repeat the earlier mentioned steps with
c:\OpenSSL-Win64\bin>openssl.exe s_client -connect smtp.example.com:25 -starttls smtp
Bonus: verify StartTLS for SMTP-, POP3- or IMAP servers – Check HTTPS TLS/SSL certificates
To verify whether your (SMTP-, POP3-, or IMAP) mail server supports StartTLS, use the following OpenSSL command:
openssl s_client -connect imap.example.com:143 -starttls imap openssl s_client -connect pop.example.com:110 -starttls pop3 openssl s_client -connect smtp.example.com:25 -starttls smtp
Check HTTPS TLS/SSL certificate
openssl to check and verify HTTPS connections:
openssl s_client -tls1_2 -servername host -connect 203.0.113.15:443
Substitute host with your host header or domain name, and 203.0.113.15 with the IP address of your web server.
Protip: check SSL certificate expiration date.
This one-liner checks the SSL certificate expiration date, from the Linux command line using
echo | openssl s_client -connect mx.example.com:25 -starttls smtp | openssl x509 -noout -dates