Microsoft 421 RP-003: too many simultaneous connections
Microsoft / Outlookconnectionaction: throttle
421 RP-003 means your address has more connections open to Outlook.com than it is allowed. Microsoft publishes a ceiling of 500 simultaneous connections and ties the practical limit to reputation below that.
Microsoft's published ceiling is 500 simultaneous connections, and almost nobody who sees this code is near it. The limit that bit is the reputation-derived one below it, which is not published.
What it looks like in the log
421 RP-003 Mail rejected by Outlook.com for policy reasonsWhy it happens
- Concurrency configured for total throughput rather than per destination, so every worker opens its own connection to Microsoft.
- Retries stacking on top of live traffic after a deferral.
What to do
- Set a per-destination connection limit for Microsoft. During a warm-up one or two is enough and costs nothing.
- Make retries share the same connection budget as new mail, rather than running alongside it.
Related
Warm-up planner sets the connection count alongside the rate, which is the pair this refusal is about. Open warm-up planner