Email Delivery Deferral

A deferral occurs when a recipient server is not ready to accept email from your IP address or domain. Instead of blocking or bouncing the message, the recipient server will defer receiving the message, and wait for the email to be retried. This guide explains how email delivery deferrals work, and recommendations for avoiding them.

Why do deferrals occur?

Deferrals may happen for a number of reasons:

  • The receiving server may be too busy to accept your messages.
  • Your IP may be throttled for reputation reasons.
  • You may have exceeded their hourly or daily rate limits.
  • There may have been a transient DNS or networking issue temporarily preventing delivery.

A deferral doesn’t necessarily mean that something went wrong; all senders receive deferrals. It is expected behavior that some messages could take more than one attempt to be successfully delivered.

What does Twilio SendGrid do when a message is deferred?

Twilio SendGrid will retry delivery of a deferred email on behalf of our customers for up to 72 hours from the time of the first deferral, After this time, the destination email address is automatically placed on your block list.

Notice: Email sent to addresses on your block list will not have their mail suppressed; a block is not considered a bounce.

What can I do to avoid my messages being deferred?

To minimize the risk of having mail deferred when sending through a dedicated IP address, we suggest you “warm” your IP by stepping up your volume over time. Mailbox providers tend to view IPs with little or no traffic with wariness, which makes it possible for them to defer your mail. As you continue to send messages over your IP, you will build that reputation and the number of deferrals may decrease.

Sending high volumes of mail (250k-500k messages per day) through a single IP can also cause deferrals due to the large amount of traffic flowing from a single IP. They best way to work around this is to purchase additional IPs to spread your traffic across. Purchasing an additional IP can be done in your Settings Menu under IP Addresses.

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