Gmail 550 5.7.29: message not sent over a TLS connection

What the bounce means and the exact steps to stop it.

The bounce looks like this
550-5.7.29 This message was blocked because it wasn't sent over a TLS connection.

What it means

Bulk senders must deliver to Gmail over an encrypted (STARTTLS) connection. Your mail server or SMTP relay sent in plain text – usually an old server, a disabled TLS setting, or an expired/invalid certificate that made the server fall back.

How to fix it

  1. Enable opportunistic TLS for outgoing mail (Postfix: smtp_tls_security_level = may).
  2. Install a valid certificate for the server's host name and keep it renewed.
  3. If an app sends through an SMTP relay, use the relay's TLS port (587 with STARTTLS or 465).

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