This post shows you how to test an StartTLS/TLS connection to an SMTP server to investigate SMTP over TLS connection issues. For this you use the OpenSSL tool (program). You also learn how to create base64 encoded logon information to verify SMTP authentication over this opportunistic TLS connection. All that from the Linux and Windows command line. This comes in very handy when investigating SMTP authentication issues, particular those over TLS encrypted connections. Investigate SMTP authentication issues like a boss using OpenSSL on your command line.

In this post you'll learn how to test SMTP authentication with StartTLS from the command promt. Neat, right?! :) This'll allow you to investigate and identify mail issues even when connections are encrypted.

SMTP Authentication

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 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 StartTLS/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 (from a website).

Before you can test the SMTP AUTH PLAIN authentication over TLS, you need to create log in information. The log in information is your username (email address) and password, normally this is an email address and its password, and a special character: \0.

Create SMTP AUTH login information

To create the logon credential combination - which has to be base64 encoded - you can use Perl and Bash in Linux, or Perl and PowerShell in Windows. In the examples I'll be using "" as the logon name, and "password" as its password.

Perl using MIME::Base64

If you're using Perl to create SMTP AUTH login information, you need to use the MIME::Base64 module:

perl -MMIME::Base64 -e 'print encode_base64("\000username\\000password")'

Don't forget to escape the @ char with a slash \: \@, otherwise it'll be interpreted as an array. The base64 encoded string will be something like:


If you're using Perl in Windows, you need to escape the double quotation (") marks like:

perl.exe -MMIME::Base64 -e "print encode_base64(\"\000username\\000password\")

Echo and base64 in Bash

You don't necessarily need Perl to generate a login hash, you can use plain old echo and base64 in Bash too:

echo -ne '\\0password' | base64

Create the SMTP AUTH login information with PowerShell in Windows

In Windows (Windows Server, Windows 11 or Windows 10), you can easily use PowerShell to create the base64 encoded login hash:


Remember the back tick (`) is your escape character in PowerShell, and not the back slash (\) like with Bash / Perl.

Connect to an SMTP server using opportunistic TLS with OpenSSL in Bash

Now you have your login hash ready, it's time to connect to an SMTP server to verify SMTP authentication over using opportunistic TLS. First you need the OpenSSL client in Linux or WSL (Windows):

sudo apt-get install openssl
sudo yum install openssl

Second, you now can use the openssl command in Bash, as explained below, to set up a TLS encrypted connection with your SMTP server:

openssl s_client -connect -starttls smtp

You may need to use a different port number like 587, ask your provider.

This p00ps out a lot of crap.. eehh verbose output, don't worry :) When the connection is made, you'll notice an SMTP 250 code:

250 DSN

This means you can start your SMTP transaction. Use EHLO to let the SMTP server print out the supported verbs:

EHLO there
250-SIZE 52428800
250 DSN

Here you notice AUTH PLAIN LOGIN as a login 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:


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.

Here's how to send authenticated SMTP email over TLS from WordPress! And how to send authenticated SMTP over a TLS encrypted connection, in PHP, ASP and ASP.NET. Neat! :)

How to install Perl in Windows

In order to accomplish all of the above on Windows Server, Windows 11 or Windows 10, you need to download and install the OpenSSL client and Perl (I use Strawberry Perl). My post Windows 11/10 and WSL 2 DevOps environment describes how you can install OpenSSL in Windows. For Perl it's just:

  • Strawberry Perl Releases (I use the ZIP edition which doesn't require an installation)
  • Unzip or and copy the folder to c:\Perl for example

Verify StartTLS for SMTP-, POP3- or IMAP servers - Check HTTPS TLS/SSL certificates (Bonus!)

As a bonus to this article: To verify whether your (SMTP-, POP3-, or IMAP) mail server supports StartTLS, use the following OpenSSL command:

openssl s_client -connect -starttls imap
openssl s_client -connect -starttls pop3
openssl s_client -connect -starttls smtp

Check HTTPS TLS/SSL certificate

Use openssl to check and verify HTTPS connections:

openssl s_client -tls1_2 -servername host -connect

Substitute host with your host header or domain name, and with the IP address of your web server.

Check SSL certificate expiration date

This one-liner checks the SSL certificate expiration date, from the Linux command line (Bash) using openssl:

echo | openssl s_client -connect -starttls smtp | openssl x509 -noout -dates
echo | openssl s_client -connect -starttls ftp | openssl x509 -noout -dates

HTH! :)

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  1. Arlé M

    If you use the command “openssl s_client -connect -starttls smtp” on a Linux box and try to connect to a Exchange server it doesn’t work. To make it work you need to add “-crlf”. Without it, it would just timeout without knowing what went wrong.

  2. Dima

    1. It is important to use -quiet switch when running openssl to avoid RENEGOTIATING error. If the command you send starts with R then s_client will renegotiate its TLS connection. Differently, if the command starts with Q then s_client will close the connection.
    c:\OpenSSL-Win64\bin>openssl.exe s_client -connect -starttls smtp -quiet

    2. There is no need to download perl on Windows machine, normally you can convert any string to Base64 using PowerShell. For example:

    • Hi Dima, thank you very much for your comment!

      I wrote the article in an era when it was not common to have PowerShell installed on your workstation, and therefore I mentioned a portable version of Perl. But great addition!

      The renegotiating error in OpenSSL is something I’ve never encountered, but it’s in the manpage: Thanks.

      • Dima

        Hi Jan,
        If your Base64-encoded password or user name will start with capital R or Q, then instead of entering password, openssl will trigger RENEGOTIATING or Quit session.
        For example:
        “Gregory” will be encoded as R3JlZ29yeQ== and trigger Renegotiation instead of sending user name to SMTP server.
        “Cpassword” will be encoded as Q3Bhc3N3b3Jk and trigger session close.
        So you were lucky that your Base64 encoded user name or password was not encoded with capital R or Q.
        This is important if you use AUTH LOGIN to test SMTP authentication.

  3. Anonymous


    Very useful. Thanks a lot.

    In windows (Power Shell) you can run the following commnad to obtain the SSL Certificate Expiration Date.(Like Linux Protip)

    write-output “quit\r” | c:\OpenSSL-Win64\bin\openssl.exe s_client -connect -starttls smtp | c:\OpenSSL-Win64\bin\openssl.exe x509 -enddate -noout

  4. Thanks so much for this! Really helped me out in troubleshooting issues with our SMTP provider (SendGrid)

    • Hi Josh, great to hear this post helped you out in troubleshooting SMTP issues with SendGrid! Spread the word ;-)

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