Connection limit
The number of simultaneous SMTP connections a sender opens to one destination, or a receiver allows from one sender, beyond which connections are refused or deferred.
What it means
Both sides set limits. Sending MTAs cap parallel deliveries per destination; in Postfix the default_destination_concurrency_limit is 20. Receivers cap how many connections they accept from one IP at a time and refuse the excess with a temporary reply: Outlook.com answers 421 RP-003 when a sending IP has exceeded its connection limit, which it ties to IP and domain reputation.
Connections are one of the levers Gmail names when a sender is rate limited: after a 4.7.28 error, resume with a single connection and add more one at a time until the desired number is reached. Matching connection counts to what each provider tolerates is part of sending at the steady rate receivers ask for.
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
- Outlook.com Postmaster: Troubleshootingsubstrate.office.com
- Google, Gmail Help: Email sender guidelinessupport.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes