When you think of email security, what comes to your mind?
Chances are, you might have thought of it as a single checkpoint where the email server decides whether to allow the email through.
But email security isn’t just about that. It’s about fighting two different battles at two different layers. The first is protecting the path that the email charts, and the other is about protecting the identity and integrity of the email.
To protect the email through its entire journey, right from the moment it leaves the sender to when it reaches its destination (the recipient’s inbox), you need encryption for the connection and authentication for the message itself.
The first half of the journey is protected by TLS (Transport Layer Security), which encrypts the connection between mail servers so the exchange stays private and harder to intercept. The second half of the journey is protected by DKIM, which adds a verifiable cryptographic signature to the email header to prove that the sending domain authorized the message and that its content wasn’t altered after sending.
In this article, we will understand how TLS and DKIM work and why you need both of them to secure your outgoing email’s journey and identity properly.
How an email travels: two checks, two layers
When you send an email, it doesn’t reach your recipient directly; the message is first processed by the sending server, which then connects to the recipient’s mail server to transfer it. The server prepares a delivery path using the email transfer rules from the protocol called SMTP.
SMTP, or Simple Mail Transfer Protocol, is a standard that tells the mail server how to relay the messages to their designated destination. But the problem is, it does not encrypt the connection, nor does it verify that the sending domain actually approved the email. That is why it is important to make extra efforts so that the email delivery is protected and the sender’s identity is verified.
To fill these gaps, it is recommended that you implement TLS and DKIM, because each secures a different risk point in delivery. Transport Layer Security protects the encrypted session between two mail servers. When your sending mail server connects to the receiving mail server, TLS encrypts the server communication and ensures that the network is not easily intercepted or manipulated.
But even when TLS encrypts the server connection, one problem still remains untackled; the receiving server still cannot confirm whether the sender’s domain was real or forged.
When you encrypt an outbound email with TLS, you are essentially only protecting the connection between the mail servers, so that the data transferred between them is not manipulated. But encryption does nothing about the legitimacy of the email, as it does not verify the From: domain in the email header or prove that the domain actually allowed the message. This means an attacker can still send a spoofed email over a TLS-encrypted connection if the message’s legitimacy is not verified. That’s where DKIM comes in.
DKIM adds that missing identity check. It creates a digital signature for the email using a private key controlled by the sending domain. When the email reaches the receiving server, the server looks up the corresponding public key in the domain’s DNS records and verifies the signature. If the signatures match, the receiving server considers the email as legitimate and delivers it to the recipient’s inbox.
Now that email-based attacks have become more sophisticated and frequent, this layered approach has become the baseline for modern mail delivery. It builds a strong and verifiable foundation for your outgoing emails.
Why can’t you secure your emails with TLS alone?
As we established earlier, there are two parts to email security. But the thing is, TLS caters to only the first part of it. It only protects the server connection while an email is being transferred. TLS only protects the connection between two mail servers when they exchange an email. It upgrades the server session into encrypted communication, so that someone on the network cannot easily intercept or tamper with the data being transferred during that connection.
While the transfer channel might be encrypted by TLS, that’s not enough.
What if you send an email, and the domain shown in the email header was never the one that approved it? TLS cannot check that. It only encrypts the conversation between servers during transfer; it does not verify whether the sender’s domain actually authorized the message.
Why can’t you secure your emails with DKIM alone?
It is only when the first half of the journey is well protected that the receiving server can trust that the email wasn’t tampered with during transit and proceed to the next step, which is domain and message verification.
When it comes to the legitimacy of the email sender and the content of the email, that’s handled by DKIM. But if the connection between the servers themselves is not encrypted, the DKIM-signed email is still exposed while moving between mail systems. The receiving server might verify a correct DKIM signature and confirm that the header fields and message body were signed by the same domain that the sender claims, but the network path used to transfer it can still be exposed. This means any attacker can intercept or interfere in the server handover before DKIM verification even begins.
So what should you do?
Clearly, email security is not a one-and-done process. It requires a layered approach so that there are no gaps left for attackers to exploit.
So, to properly protect your emails, make sure that you pay attention to both server connection security and email authentication. With both TLS and DKIM in place, you can ensure two things before delivery: the email moved through an encrypted server connection, and it reached its destination without any tampering. Only when your email checks both boxes can the receiving server trust your email.
Want to know how to implement TLS and DKIM for your domain? Contact us today!
