Definition
Delayed delivery, also called a soft bounce or deferral, happens when a receiving mail server responds with a 4xx SMTP error code indicating a temporary condition — such as rate limiting, a full recipient inbox or a greylisting check. The sending server queues the message for retry rather than treating it as a permanent failure.
Unlike a hard bounce which results in immediate non-delivery, delayed messages are attempted again at increasing intervals. Most reputable sending platforms retry for 12 to 72 hours before marking the message as permanently failed.
Common Causes of Delayed Delivery
- Greylisting defences on the recipient server
- Rate limiting because the sender is delivering too many messages at once
- Temporary DNS lookup failures
- Recipient's inbox is over quota
- The recipient server is undergoing maintenance
Best Practices
- Review delayed delivery rates by recipient domain to identify patterns
- If delays are concentrated at a specific provider, investigate throttling or reputation issues
- Ensure your sending pattern does not exceed rate limits set by major mailbox providers
- Treat sustained elevated delay rates as an early warning signal for broader deliverability problems
Related Glossary Terms
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
DMARC Alignment
DMARC identifier alignment determines whether the domain in the From header matches the domains used in SPF and DKIM authentication. Strict or relaxed.
Acceptance Rate
Acceptance rate is the percentage of emails submitted for delivery that are accepted by the recipient's mail server, representing the first deliverability checkpoint before inbox placement is determined.
Email Acceptance Rate
Email acceptance rate measures the percentage of emails accepted by recipient mail servers versus total sent. It is a core deliverability metric that reveals whether your sending infrastructure is trusted by receiving servers.