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SMTP sending is throttled at two points: authentication attempts (AUTH PLAIN, per sending domain) and send rate, the number of messages accepted per second across your whole organization, counting every domain and SMTP credential together. REST sends are throttled against the same per-second value in a separate window, so the two channels do not consume each other’s.

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Authentication rate

Each sending domain is limited to 6 AUTH PLAIN attempts per second. Exceeding it returns 454 4.7.0 Authentication temporarily throttled, please retry later to the AUTH command and leaves the session open. Back off before you retry AUTH. The limit protects against a misconfigured or looping mail client repeatedly retrying auth.

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Send rate

Your per-second rate is set by your plan, from 4/sec on Free up to 10/sec on Scale, Volume, and Enterprise. See Plans & limits for the value on each tier. Accounts with no explicit entitlement fall back to 1/sec. When you exceed the rate, the submission is rejected with 450 4.7.1 Rate limit exceeded, please retry later; retry once the one-second window turns over. That throttle is distinct from your monthly quota, and from the daily one on the plans that have it: an exhausted quota is a permanent 550 5.7.0 until its window resets. See Error codes.

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Staying within the limit

  • Let your mailer queue messages and pace delivery over a few long-lived connections, within the connection limits.
  • If you consistently need a higher rate, upgrade your plan.