Anything you include in emails is private and should only be seen by the recipient. You can send social security numbers, business reports, login information, kid-friendly recipes, or photos

But if you want to make a wise choice or if you're just setting up your client, it's critical to comprehend what happens behind the scenes. What is SMTP security? Read on to find out.

What is SMTP?

Email clients use the Simple Mail Transfer Protocol technology to transfer messages from one server to another and to end users. In a nutshell, it sends your emails to the recipient in a matter of seconds.

SMTP handles a significant portion of every outgoing transmission. However, the default SMTP protocol lacks security features, making it open to hijacking and other types of attacks.

What are the threats to SMTP security?

When sending even a small number of emails, there are a few things to be mindful of. Here are some of the most frequent ones:

  • Cybercriminals using less-than-sophisticated techniques to get around your authentication procedures can give themselves access to your emails and cause data leakage.
  • Fraudsters use your SMTP server to send unauthorized messages to your contacts and external accounts, a practice known as spamming or phishing.
  • Attackers use malware to infect your infrastructure and the recipients of your emails. These might take the form of viruses, Trojan horses, or other worms that obstruct operations, access servers, change privileges, and access secure data.
  • Cybercriminals may launch Denial-of-Service (DoS) attacks using your SMTP server. In essence, this entails flooding other servers with emails to overwhelm them and possibly bring about a crash.

How to Make SMTP secure? What's SSL/TLS?

SSL (Secure Sockets Layer) and TLS (Transport Layer Security) were added to the SMTP protocol suite to improve security. The terms "SSL" and "TLS" are frequently used synonymously, so a service provider might be referring to SSL when they are actually using TLS.

Email messages sent back and forth between your email client and email server can be encrypted using SSL/TLS. As an additional security measure, TLS also supports the use of digital certificates. The term "Handshake" refers to this action.

How Does the Handshake Work?

A handshake is when an email transitions from one of these stages to the next, and a connection is made using TLS. But both parties must establish trust with one another.

This communication takes place between the client of the sender and the server of the recipient to guarantee the accuracy of the information it is sharing. They won't start transmitting until they have reviewed each other's papers and decided on some ground rules for cooperation.

The interactions with encryption between the two servers initially happened asynchronously. They use a shared secret key during the handshake, making the encryption synchronous.

Opportunistic vs. Forced TLS. STARTTLS

An email client will inform an email server that it wants to speak with them privately when using Opportunistic (Explicit) TLS during a Handshake. It suggests switching from an unencrypted, plain SMTP connection to one that is TLS-encrypted.

An email client will request that they communicate privately (and over an encrypted connection) when using Forced (Implicit) TLS. If the email does not support TLS at all, is incompatible with the client's version of TLS, or the connection fails, the transmission will be terminated, and the email will not be further processed.

When using Opportunistic, the SMTP command STARTTLS is used to start the transition to an encrypted connection.

End-to-end encryption

End-to-end encryption is a different approach for safeguarding email communication. A message is encrypted on the sender's device before it leaves the client on the server. It then moves through the network while frequently being TLS-encrypted along the way for added security. Once it gets to the recipient's client, the recipient decrypts it.

The most popular end-to-end encryption methods are:

  • S/MIME: Secure/Multipurpose Internet Mail Extensions are one of the popular encryption techniques. It is based on a set of public and private keys and asynchronous encryption.
  • PGP: Since the early 1990s, Pretty Good Privacy has been the most widely used encryption technique. Along with emails, it can also encrypt files, directories, and entire disk partitions.
  • Bitmessage: The phrase "bitcoin of communications" frequently refers to Bitmessage. Here, users must create a Bitmessage address (a pair of public and private keys) before they can send messages.

Additional Information Regarding End-to-End Encryption

End-to-end encryption improves the deliverability of your emails and stops spoofing of your communications.

SPF, DKIM, and DMARC are the most widely used techniques. SPF uses the IP addresses used to send emails on behalf of a particular domain to upload them to the DNS records, allowing recipients to identify the sender. On the other hand, DKIM is a digital certificate that is attached to emails. It enables the recipient of an email to confirm that the content or headers of the email were not altered (faked) during transmission.

The most complex of the three techniques, DMARC performs additional checks using the other two techniques. It's the only method that, except for running a test, can also suggest to a receiving server what to do if a message fails a check.


SMTP is a trustworthy protocol that permits secure email transmission because it creates a connection with the recipient's server, establishes communication, and sends the message back to the sender.