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Outlook.com 421 RP-001 / RP-002 / RP-003 — Rate Limited

Outlook.com is temporarily throttling the sending IP: it exceeded the allowed rate, connection count, or is being rate limited for reputation while history accumulates.

Diagnose your exact bounce Email Infrastructure Digital Twin

What the bounce says

Exact wording as it appears in the rejection or NDR. Placeholders such as x.x.x.x and example.com stand for your own address and domain.

421 RP-001 The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation problems.

421 RP-002 The mail server IP connecting to Outlook.com server has exceeded the rate limit allowed on this connection. Reason for rate limitation is related to IP/domain reputation problems.

421 RP-003 The mail server IP connecting to Outlook.com server has exceeded the connection limit allowed.

RP codes are temporary throttles. RP-001 and RP-002 say the IP exceeded the rate Microsoft allows given its reputation; RP-003 says it opened too many connections. All three clear as the rate falls and reputation builds.

Reviewed 2026-09-08. Provider wording and requirements change; the provider reference below is authoritative.

Most common causes

  • Too many messages or connections in a short window
  • A new IP sending at full volume without warm-up
  • Reputation not yet established for the IP
  • A burst after a period of low activity

What to verify next

  1. Let the queue retry with backoff; do not open more connections
  2. Reduce hourly volume and concurrent connections to Microsoft
  3. Warm new IPs gradually
  4. If persistent, check reputation via SNDS and sender support

Rate is relative to reputation

A well-established IP can send far faster than a new one. The same volume that is fine from a warmed address triggers RP-001 from a fresh one. Microsoft adjusts the ceiling as history accumulates.

RP-003 is purely about concurrency: too many simultaneous connections from one IP. Reducing parallelism fixes it without changing volume.

  • New IPs have low ceilings.
  • RP-003 is concurrency, not volume.
  • Queues should back off, not retry immediately.

Configure the sender

Limit concurrent connections to Outlook.com destinations, add per-domain rate limits in the MTA, and spread large sends over hours. Most MTAs and platforms have per-destination throttling settings for exactly this.

Build reputation

Warm new IPs over one to two weeks, send to engaged recipients first, keep authentication aligned, and enrol in SNDS to watch the ceiling rise.

Best diagnostic path

Live analysis

Mail Failure Doctor

Classify the complete rejection and preserve provider-specific diagnostic context.

Open analysis → Live analysis

Email Infrastructure Digital Twin

Map the domain’s public mail infrastructure and provider relationships before remediation.

Open analysis →

Known limits

  • Ceilings are not published and vary per IP.
  • Throttles are temporary but recur if volume stays above the ceiling.

Common questions

Is mail lost?

No. 421 asks the sender to retry; the queue delivers as the throttle lifts.

How many connections are allowed?

Microsoft does not publish a number. Start low — a handful — and increase as RP-003 stops appearing.

Does a dedicated IP help?

It gives you control over reputation, but a new dedicated IP starts with a low ceiling and needs warm-up.

Why the exact message matters

The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.

Provider reference

For the provider-defined meaning and current requirements, review Microsoft Outlook.com sender troubleshooting.