Deferrals

A preemptive deferral occurs when an email delivery system temporarily postpones sending messages because it anticipates that immediate delivery would violate recipient server policies or ISP limits, such as throttling, hourly limits, or connection caps. This proactive approach helps prevent delivery failures and ensures compliance with external sending restrictions. We protect the sender's reputation by returning a preemptive deferral with the reason for the delay at this time and will retry delivery for up to 72 hours. Some preemptive deferrals include actions to help the sender avoid similar issues in the future.

You might see the following error messages with a deferral event in your logs:

• Email was deferred due to the following reason(s): ["IPs reached ISP-suggested hourly limits: https://www.twilio.com/docs/sendgrid/ui/sending-email/deferrals"]
  • This indicates you've exceeded IP warmup limits on a new IP. Learn more about Warming up an IP address.
• Email was deferred due to the following reason(s): ["IPs reached ISP-suggested max connection limits: https://www.twilio.com/docs/sendgrid/ui/sending-email/deferrals"]
  • This deferral is created by SendGrid to control the rate of delivery by ISP to help prevent throttling and spam folder delivery. Sending too much mail too quickly can trigger suspicion from email providers and harm deliverability. This automatic deferral allows you to send large volumes at once while protecting your deliverability.
• Email was deferred due to the following reason(s): ["IPs reached max port limit: https://www.twilio.com/docs/sendgrid/ui/sending-email/deferrals"]
  • This deferral is generated by SendGrid when your sending IPs have reached the maximum number of ports per IP. To reduce the number of these deferrals, you should add additional IP addresses to your account.
• Email was deferred due to the following reason(s): ["IP reached max connection limit: https://www.twilio.com/docs/sendgrid/ui/sending-email/deferrals"]
  • This deferral is generated by SendGrid when an IP has reached the maximum number of connections. To reduce the number of these deferrals, you should add additional IP addresses to your account.
• Email was deferred due to the following reason(s): ["IPs were throttled by recipient server: https://www.twilio.com/docs/sendgrid/ui/sending-email/deferrals"]
  • This deferral indicates that the recipient server recently refused an email likely due to reputation issues or sending mail too quickly. We returned a preemptive deferral to protect your sender reputation.

Delivery was attempted but there is an internal issue with our system. We'll retry for up to 72 hours.

The response will contain "Internal Server Error" in this case. There is no action required from the sender.

Delivery was attempted, but the recipient's mailbox provider has requested that the sender slow down or try again later. We'll retry delivery for up to 72 hours.

The response will include a "reason" field with an SMTP response code (for example, 450), an enhanced status code (for example, 4.2.2), and a short explanation (for example, Mailbox full). Depending on the response, there might be actions required from the sender.