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. This process of intentionally limiting the amount of emails sent through a server in a limited amount of time is known as throttling.

Definitions

Here's some terms you will come across in this article.

• Throttling: Intentionally limiting the amount of emails sent through a server in a limited amount of time.
• ISP: Internet service provider
• IP address: A unique set of numbers that help identify a specific user or a user's location while on the internet.
• Spam traps: Email addresses used by ISPs to find people who send emails to out-of-date contacts or contacts they don't have permission to send to.

How it works

A number of factors affect which campaigns ISPs throttle, and for how long. Here’s a few factors that may be used to determine what limits each ISP uses.

• 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.

Handling new IP addresses

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.