What is email throttling?

Email throttling is a defensive measure where a receiving mail server intentionally limits the rate at which it accepts messages from a specific sender, sending IP, or sending domain. Common in two scenarios: (a) the receiver suspects you might be a spammer and wants to slow you down, or (b) the receiver protects its own infrastructure from being overwhelmed by any one sender.

Throttling shows up in three ways:

• Soft bounces (4xx codes). Receiver returns a temporary failure instructing you to retry later.
• Slower delivery. Messages accepted but processed at a deliberately reduced rate.
• Connection limits. Receiver caps the number of simultaneous SMTP connections from your IP.

Triggers:

• Sudden large increase in volume from a previously low-volume sender.
• Sender on a new IP without proper warm-up.
• Recent uptick in bounce rate or complaint rate.
• Sender appearing on a blacklist consulted by the receiver.
• Time-of-day patterns inconsistent with legitimate senders.

Mitigation: warm up new IPs/domains gradually, send consistently rather than in big bursts, maintain clean lists via verification, fix authentication. Throttling is reversible — once your reputation improves, the rate limits relax automatically.