451 4.7.1 — Ratelimit exceeded
You are being rate limited, not blocked
SMTP error 451 4.7.1 with a "Ratelimit" message is a temporary throttle from your own outbound host, telling you to slow down. The 4.x class means try again later. It is not a bounce, not a reputation problem, and not a statement about the recipient - the host simply refused this message at this moment.
What the server actually says
451 4.7.1 Ratelimit "hostinger_out_ratelimit" exceeded for key "u123456789"
please try again laterWhat you see
A burst of sends succeeds, then a run of failures, then recovery. Volume looks sawtoothed rather than flat, and the failures cluster at whatever time of day sending is densest.
Why it happens
- Sending faster than the host’s per-minute or per-hour outbound rate, even while well under any daily cap.
- Shared hosting plans apply a single rate key across every mailbox on the account - one busy mailbox throttles the others.
- No spacing between sends, so a tick dispatches many messages in the same second.
- A provider that advertises a generous daily limit but enforces a much tighter burst limit.
How to fix it
- Space the sends out. Add jitter between messages so a scheduler tick does not dispatch its whole batch at once. Human senders do not send ten messages in one second.
- Find the burst limit, not the daily limit. Hosts publish daily caps and enforce per-minute ones. The per-minute number is what produces 451 4.7.1.
- Check whether the rate key is shared. On shared plans the key covers the whole hosting account, so the fix is reducing total concurrent sending, not per-mailbox volume.
- Let the send retry later instead of failing it. A 4.x is explicitly retryable. Park the message and try again rather than recording a failure against the mailbox.
What not to do
- Do not count this as a sender fault. "Slow down" is not "this sender is broken", and treating it as one can auto-stop mailboxes that are performing perfectly.
- Do not suppress the recipient. The 4.7.1 enhanced code matches naive soft-bounce rules, but the recipient was never evaluated.
- Do not escalate volume to "catch up" after the throttle clears. That guarantees hitting it again, harder.
Frequently asked questions
Is 451 4.7.1 a bounce?
No. The 4.x class means temporary - the correct response is to retry the same message later. Bounces are 5.x and mean permanent.
Why does the message mention a key?
Shared hosts meter outbound mail against an account-level rate key rather than per mailbox. Every mailbox under that key draws from the same allowance.
Does getting rate limited hurt my domain reputation?
No. The throttle happens at your own outbound host before the mail reaches any receiving provider, so no inbox provider ever sees it.
Can three of these in a day mean my mailbox should stop sending?
It should not. A throttle says nothing about sender health, and stopping on it takes a healthy mailbox out of its ramp for no reason.
How MailPilot helps
MailPilot classifies this error automatically on every connected mailbox and tells you which side is at fault, so a throttle is never mistaken for a dead recipient and a credential failure never suppresses a healthy address. It also monitors real inbox placement and checks SPF, DKIM and DMARC on every mailbox, which is where most policy rejections start.
Related errors
- 550 5.4.5 — Daily user sending limit exceeded
- 421 4.7.0 — Too many messages - try again later
- Too many simultaneous connections — IMAP connection limit reached
Last reviewed 2026-10-11 by the MailPilot team.