Infrastructure
What is TLS (Transport Layer Security)?
The encryption protocol that secures email in transit between mail clients and servers.
TLS encrypts the connection between mail clients and servers, and between mail servers, so message contents and credentials cannot be read in transit. For email it is negotiated either implicitly on a dedicated port such as 465, 993, or 995, or opportunistically via STARTTLS on a standard port. It is the successor to SSL and underpins enforcement mechanisms like MTA-STS and DANE.
Why it matters for cold email
Mailbox providers increasingly expect and reward TLS-encrypted delivery, and Google Postmaster Tools even reports your TLS rate. Failing to send over TLS can weaken trust signals, and with MTA-STS in play it can cause hard delivery failures that silently kill a cold campaign.
Related terms
Stop guessing at deliverability.
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