TEmail Deliverability Glossary

TLS (Transport Layer Security)

The encryption protocol securing email in transit between mail servers. Required by Google's 2024 sender guidelines and monitored in Google Postmaster Tools.

TLS is the encryption protocol used to secure email in transit between mail servers (SMTP over TLS). When email is sent with TLS, the contents of the message are encrypted as they travel between servers, preventing interception.

Why TLS matters for deliverability:

Google's 2024 sender guidelines require TLS encryption for bulk senders
Postmaster Tools reports your TLS compliance rate
Not using TLS is increasingly a negative signal to modern filtering systems

How to enable: Most modern ESPs and mail servers support STARTTLS or SMTPS by default. Ensure your sending server is configured to use TLS for all outbound SMTP connections.

Frequently Asked Questions

Does email encryption (TLS) affect inbox placement?

TLS doesn't directly determine inbox placement, but it has become an increasingly significant trust signal. Google's 2024 bulk sender guidelines explicitly require TLS for all email sent to Gmail users. Google Postmaster Tools tracks and reports your TLS compliance rate. Sending without TLS to Gmail is now treated as a negative signal that contributes to lower trust scoring. Most modern ESPs and mail servers support STARTTLS by default, so TLS encryption is typically automatic — but verify your TLS compliance rate in Postmaster Tools to confirm it's near 100%.

What is STARTTLS and how is it different from SMTPS?

STARTTLS is an opportunistic upgrade command — a connection starts unencrypted on port 25 (or 587) and then both servers negotiate to upgrade to an encrypted TLS connection before any email content is transmitted. SMTPS is a dedicated encrypted SMTP channel on port 465 that requires TLS from the moment of connection. Both result in an encrypted email transit channel, but they work differently. STARTTLS is the more common standard for server-to-server email delivery. For email client-to-server connections (your email client sending through your ESP), port 587 with STARTTLS or port 465 with SMTPS are both acceptable.

How do I check if my emails are being sent with TLS encryption?

Two methods: First, Google Postmaster Tools shows a 'Delivery Errors' and encryption section that reports TLS compliance percentage for your Gmail-bound sends — look for TLS rates near 100%. Second, send a test email to a Gmail address, open it, click the down-arrow next to the recipient name in the email header, and look for 'Standard encryption (TLS)' in the security section. If you see 'No encryption,' your sending infrastructure is not establishing TLS connections to Gmail's servers, which requires investigation with your ESP or mail server administrator.

Related Terms

Authentication (Email Authentication)

The set of technical protocols (SPF, DKIM, DMARC) that verify a sender's identity and prove an email genuinely originated from the claimed domain.

Email Infrastructure

The complete technical stack for sending and receiving email — servers, IPs, DNS, authentication protocols, and monitoring tools.

Deliverability (Email Deliverability)

The ability to successfully land emails in the recipient's primary inbox — not spam, not promotions, not blocked. The ultimate goal of every warm-up effort.

SPF (Sender Policy Framework)

An authentication protocol that specifies which mail servers are authorized to send email for your domain. Published as a DNS TXT record checked on every inbound message.

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