What is SMTP TLS Report?

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1. Introduction
As businesses increasingly rely on email as a primary means of communication, the importance of hardening these channels against potential threats cannot be overstated. Transport Layer Security (TLS) ensures the confidentiality and integrity of data transmitted across networks. There are several protocols that help encrypt the SMTP message channel, preventing network attackers from intercepting email communications. These include STARTTLS, DANE, and MTA-STS. However, when using these protocols, your email may not be delivered if the encryption attempt fails. TLS-RPT (described in) provides a feedback mechanism to report these delivery failures. We strongly recommend using TLS-RPT with the protocols. Let's take a look at how these protocols work together to strengthen email security.
2. TLS-RPT (Transport Layer Security Report)
TLS-RPT (Transport Layer Security Reporting) is a standard for reporting phone number list email delivery issues when emails are not encrypted using TLS. Its importance in email authentication goes hand in hand with the reasons to enable TLS encrypted emails. TLS encryption technology ensures that every message sent to you is delivered securely. If the connection is not secure, many times emails may not be delivered. With TLS-RPT, domain owners can monitor email delivery and connection failures. Reports can include the following information so you can understand your email pipeline and address deliverability challenges faster.
3. SMTP Email Encryption
In SMTP email communication, TLS encryption is "opportunistic". This means that if an encrypted channel cannot be negotiated, the email will still be sent in an unencrypted (plain text) format. In fact, nearly 40 years ago, the SMTP email protocol did not support TLS encryption. It was later transformed in the form of the STARTTLS command. The STARTTLS command is only issued if both parties in the SMTP communication support TLS encryption. Otherwise, the email will still be sent in plain text.
4. MTA-STS
To get rid of opportunistic encryption in SMTP, MTA-STS () was introduced. The MTA-STS protocol ensures that the message is encrypted before it is sent. The email server or mail transfer agent (MTA) negotiates with the receiving server to see if it supports the STARTTLS command. If it does, the email is encrypted via TLS and delivered. Otherwise, the delivery fails. There can be several reasons why TLS encryption fails. In addition to neither party supporting encryption, more nefarious reasons such as SMTP downgrade attacks can cause the TLS connection to fail. If MTA-STS is enabled, the attacker cannot send information in plain text when the connection fails.