Send throttling
A sender deliberately limiting how fast its servers deliver mail to each destination, so the pace stays within what each mailbox provider accepts.
What it means
Receivers rate-limit senders that go too fast, answering with temporary 4xx failures; M3AAWG lists throttling of the connecting IP among the causes of soft bounces, and Gmail's guidelines advise a consistent sending rate without bursts. Send throttling is the sender's side of that: pacing delivery before the receiver has to.
Mail servers have settings for it: Postfix's default_destination_rate_delay inserts a delay between deliveries to the same destination, and its concurrency limits cap simultaneous connections per destination. M3AAWG expects senders to adjust to temporary failures in real time across all mail to the same receiver.
Set the pace per provider: a rate one large provider accepts may be too fast for a smaller system.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Postfix configuration parameters (postconf.5)postfix.org
- M3AAWG Sender Best Common Practices, version 4.0 (August 2026)m3aawg.org
- Google: Email sender guidelinessupport.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes