Message Failures
When an email fails to deliver, the outcome depends on whether the failure is permanent or temporary.
Permanent failures
Permanent failures are not retried. Common examples include:
- The recipient address does not exist
- The recipient server has blocked delivery
- The recipient’s mailbox is full
- Any 5xx error returned by the recipient server
Temporary failures
Temporary failures are automatically retried according to the schedule below. Common examples include:
- Message deferrals, throttling, or rate limiting by the recipient server
- Transient DNS issues on the recipient's domain
- Technical issues on the recipient’s mail server
Delivery Retry Schedule
Messages that encounter temporary failures are retried at increasing intervals for a maximum of 8 hours. Delivery attempts continue until the message is successfully delivered, a permanent failure occurs, or the retry window expires.
The retry intervals are:
- 1st temporary failure → retry after 10 minutes
- 2nd temporary failure → retry after 15 minutes
- 3rd temporary failure → retry after 30 minutes
- 4th temporary failure → retry after 1 hour
- 5th temporary failure → retry after 2 hours
- 6th temporary failure → retry after 4 hours
This produces a default retry window of up to 8 hours.
If a message still cannot be delivered after the full 8-hour retry schedule, it is dropped will not be delivered.