Microsoft 421 RP-001: rate limited by Outlook.com
Microsoft / Outlookrate_limitedaction: throttle
421 RP-001 means Outlook.com accepted the connection and then refused to take more mail from your address for now. Microsoft ties this to reputation rather than to a published number, so there is no volume you can stay under to avoid it.
The common advice is to wait and retry, and that is only half right. A 421 that repeats at the same volume is not a queue problem, it is Microsoft telling you the address has not earned that rate. Waiting without lowering the rate produces the same deferral tomorrow.
What it looks like in the log
421 4.7.0 [TSS04] Messages from x.x.x.x temporarily deferred421 RP-001 Mail rejected by Outlook.com for policy reasonsWhy it happens
- Sending faster than the reputation of the address supports, which is the usual cause on a new or recently quiet address.
- A volume increase that arrived as a step rather than a ramp.
- Complaints from Outlook, Hotmail or Live recipients pushing the address into a band where less mail is accepted.
What to do
- Lower the hourly rate to Microsoft specifically rather than the daily total. The limit is applied per connection and per hour, so a daily quota spread evenly is what clears it.
- Let the queue drain before sending more. Retrying immediately is what turns a deferral into a block.
- Enrol in SNDS and JMRP so there is data behind the next change.
- If this started when volume grew, the ramp was too steep. Rebuild it gated on the metrics rather than on the calendar.
Related
RP-002 · RP-003 · 4.7.500 · 5.7.606
Warm-up planner sets the hourly rate per address rather than a daily total, which is the control this deferral responds to. Open warm-up planner