471 is not defined in RFC 5321 and is not issued by Gmail, Microsoft 365, or Yahoo's consumer mail infrastructure. It originates specifically from the legacy Microsoft IIS SMTP service — a self-hosted Windows mail relay component, historically common on smaller business servers — as a vendor-specific extension code, typically meaning the sending IP tripped a local error-rate limit configured on that particular server.
Who is at fault
Context-dependent, but since this code comes from a specific self-hosted relay rather than a major provider, it usually means your sending behavior toward that one small/legacy mail system triggered a local protective threshold the server administrator configured — too many errored transactions in a short window.
The exchange
220 legacy-mail.example.com Microsoft ESMTP MAIL Service ready EHLO mail.yourdomain.com 250 legacy-mail.example.com MAIL FROM:<sender@yourdomain.com> 250 2.1.0 Sender OK RCPT TO:<recipient@example.com> 471 There are too many errors from your IP, please try later (IIS SMTP Service, non-RFC extension)
How the big three handle the same underlying concern
| Provider | Equivalent real-world condition | Typical wording |
|---|---|---|
| Gmail | 421 4.7.28, reputation-based rate limiting | Unusual rate of unsolicited mail |
| Microsoft 365 | 421 4.7.500, server busy / too many connections | Server busy |
| Yahoo/AOL | 421 4.7.0, complaint-based deferral | Try again later |
None of the three major providers use base code 471 — if you're seeing it, the receiving system is very likely a smaller, self-hosted or legacy mail server, not one of these.
Diagnose it
$ dig MX example.com +short $ host $(dig MX example.com +short | head -1)
Checking the MX record and reverse-resolving the host often confirms you're dealing with a small or legacy mail system rather than a major provider's infrastructure — useful context before deciding how much remediation effort the situation warrants.
Fix it, in order
- Reduce the error rate you're generating against this specific server — stop sending to any known-bad addresses on that domain first, since repeated failed RCPT attempts are the most common trigger.
- Clean your list for that specific domain before resuming — this code is frequently triggered by a batch containing several invalid addresses at the same small domain in quick succession.
- Space out retries rather than hitting the same server repeatedly in a short window — legacy IIS SMTP rate limits are often quite aggressive by modern standards.
- Contact the domain's administrator directly if this is a known business contact and the condition persists — small self-hosted servers often have a human you can actually reach.
Why you're increasingly unlikely to see this at all
It's worth knowing the broader context: Microsoft formally deprecated the standalone IIS SMTP service years ago, and it has not shipped as an actively developed component in current Windows Server releases for some time — most servers still running it are long-lived legacy installations that were never migrated to a modern mail transfer agent or a hosted email platform. If you're seeing 471 with any regularity across many different recipient domains rather than one or two specific legacy contacts, double-check that you're reading the raw reply correctly, since a genuinely widespread pattern is more likely to indicate a different, mislabeled condition than a sudden proliferation of legacy IIS servers.
An additional diagnostic step
$ nmap -p25 --script smtp-commands mx.example.com
Banner-grabbing the receiving server directly (where you have a legitimate reason to probe it, such as diagnosing your own delivery failures to a known business contact) confirms the server software and version, which settles definitively whether you're dealing with a legacy IIS relay or a modern system issuing a superficially similar message.
How long the local rate-limit condition typically lasts
Unlike a major provider's reputation-based throttle, a legacy IIS SMTP rate limit is usually configured with a short, fixed local window — commonly resetting within minutes to a few hours once your error rate against that specific server drops. There's no broader reputation accumulation across other recipients the way there is with 4.7.28-style deferrals from major providers; the condition is entirely local to that one server's own counters, and clears on its own timeline regardless of anything happening with your sending reputation elsewhere.
What not to do
- Don't look for this code in Gmail, Microsoft 365, or Yahoo's own documentation. It won't be there; none of them issue it.
- Don't treat it as evidence of a broader reputation problem. It's specific to one small server's local configuration, not a signal about your standing with major mailbox providers.
- Don't assume modern delisting or reputation-repair advice applies. This is a local rate-limit on one legacy system, resolved by reducing your error rate against that specific server, not by any blocklist-style remediation.
MailPilot's list hygiene checks catch the kind of invalid-address clustering that typically triggers legacy rate-limit responses like this one, before a batch send runs into them.
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