Microsoft 421 RP-002: too much mail on one connection
Microsoft / Outlookrate_limitedaction: throttle
421 RP-002 means Outlook.com took as many messages as it will accept on that single connection. Microsoft attributes it to reputation, the same as RP-001, but the unit being limited is the connection rather than the address.
RP-001 and RP-002 look identical in a log and have opposite fixes: one wants fewer messages per hour, the other wants fewer messages per connection. Changing the wrong one moves nothing.
What it looks like in the log
421 RP-002 Mail rejected by Outlook.com for policy reasonsWhy it happens
- Too many messages pipelined down one connection before closing it.
- A pool that opens few connections and drives all volume through them.
What to do
- Cap messages per connection and let the MTA open a fresh one. Most MTAs expose this separately from the rate limit and it is often left unbounded.
- Do not answer this by opening more connections at once. That is the next code down, RP-003.
Related
Warm-up planner gives a rate per address and per provider, so the volume going to Microsoft can be brought down on its own. Open warm-up planner