How Throttling Improves Deliverability

Mailchimp aims to deliver your campaigns as quickly as possible. Technically, our system can deliver one million emails in 45 minutes, but major ISPs batch and postpone delivery as a means of preventing spam. A number of factors affect which campaigns ISPs throttle, and for how long.

There's no public, authoritative source on what limits each ISP uses, but there are a few factors involved.

  • bounce rates
  • abuse complaints
  • subscriber engagement, including open rates, click-throughs, and time spent viewing emails
  • spam trap hits

The limits applied by major ISPs can also change if suspicious behavior is detected. When an ISP sees five bounces within a certain timeframe, they may send a throttling message that tells the sending server to slow down or stop sending for a certain period of time. If that message is ignored, the ISP could escalate to blocking mail from that IP address altogether.

For new IP addresses, ISPs will generally throttle the amount of mail from that IP address until they get an idea of the type, volume, and overall characteristics of the mail being sent.

Consistency and quality are the key factors, so a new IP address will need to send mail for some extended period of time and in a consistent manner before the ISP will allow more mail per hour or per day.

If you're sending to one of the major ISPs that throttles emails, Mailchimp will send your campaign from multiple IP addresses to account for throttling. There are no boxes to check or settings you need to change; this all happens automatically on our servers.

With that in mind, it isn't possible to introduce artificial delays into the send process, or into a specific user's campaigns. Likewise, for corporate or personal domains, it's not feasible to specify how quickly we send to those domains.

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