Infrastructure
What is STARTTLS?
A command that upgrades a plaintext email connection to encrypted TLS on the same port.
STARTTLS is an opportunistic encryption mechanism: a client or server begins on an unencrypted port (such as 587 for submission or 143 for IMAP) and issues the STARTTLS command to negotiate TLS before exchanging credentials or mail. Because it starts in cleartext, it is vulnerable to STARTTLS-stripping downgrade attacks unless enforced by a policy like MTA-STS or DANE. It contrasts with implicit TLS, which is encrypted from the first byte.
Why it matters for cold email
Cold-email sending and inbox-sync connections typically rely on STARTTLS on submission and IMAP ports, so misconfigured or stripped STARTTLS can cause auth failures or unencrypted delivery that some receivers penalize. The result is sends and reply sync that break quietly without an obvious error.
Related terms
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