Rate limiting
A receiving provider limiting how fast it accepts mail from a sender, answering excess connections or messages with temporary 4xx replies so they arrive later.
What it means
Mailbox providers cap how much mail they take from an IP, a domain or an account in a period, and set the caps lower for senders with little history or a poor reputation. Google states that Gmail tracks volume, feedback and limits per domain and per IP address, and that it typically answers a sender over quota with SMTP error 4.7.28. Microsoft answers with 4.7.500 to 4.7.699 when it temporarily restricts sending after detecting suspicious activity, Outlook.com 421 RP-001 and RP-002 for rate limits tied to reputation, and Yahoo its TS deferral codes.
The remedy is to slow down. After a 4.7.28 error Google advises stopping for at least 10 minutes, resuming on one connection and adding connections one at a time, and more generally sending at a steady rate, increasing volume gradually and avoiding bursts.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
- Outlook.com Postmaster: Troubleshootingsubstrate.office.com
- Yahoo Sender Hub: SMTP error codessenders.yahooinc.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes