Summary
The 'Resources restricted - exceeds recipients limit' bounce message (often a 451 error) signals that the recipient server perceives the sending rate or volume as excessive. This can be due to the receiving server experiencing high traffic or resource constraints, rate limiting imposed by the recipient, greylisting, poor IP reputation, or problems with sender identification (SPF, DKIM, DMARC). Some domains, like 'optonline', may require extreme sending limit reductions. As defined in RFC specifications, 4xx errors represent temporary failures, suggesting a retry. Accurate interpretation of bounce messages necessitates investigating sender reputation, content, and infrastructure. Additionally, server-side throttling policies (e.g., in Microsoft Exchange and Amazon SES) are in place to manage resources and prevent server overload. Key factors also include the warming of IPs, monitoring of data reliability, and compliance with security policies.
Key findings
- Rate Limiting: The sending rate is exceeding recipient server limits.
- Temporary Deferral: 4xx errors indicate temporary issues, suggesting a retry.
- Server Overload: Recipient server may be temporarily overloaded or lacking resources.
- IP Reputation: Poor IP reputation can trigger rate limits and 451 errors.
- Greylisting: Recipient-side greylisting can temporarily reject unknown senders.
- Domain Specific Issues: Specific domains may require drastic sending limit reductions.
- Throttling Policies: Recipient servers use throttling policies to manage resources.
Key considerations
- Sender Identification: Ensure proper SPF, DKIM, DMARC configuration.
- IP Warming: Gradually warm up new IPs to avoid triggering limits.
- Sending Volume: Adjust sending volume to respect recipient limits.
- Retry Strategy: Implement a retry mechanism for deferred messages.
- Reputation Monitoring: Monitor and improve sender reputation.
- Error Analysis: Accurately interpret bounce messages and SMTP status codes.
- Data Reliability: Ensure the reliability of delivery reports.
What email marketers say
9 marketer opinions
A '451' error message, often accompanied by 'Resources restricted - exceeds recipients limit', indicates a temporary issue preventing email delivery. This can stem from various factors including the recipient server experiencing high traffic or resource constraints, rate limiting imposed on the sender, greylisting techniques, poor IP reputation, or issues with sender identification (SPF, DKIM, DMARC). Proper IP warming and gradual sending volume increases are also essential.
Key opinions
- Server Overload: The recipient server may be temporarily overloaded, lacking the resources to handle incoming connections.
- Rate Limiting: The sending IP or domain might be exceeding rate limits imposed by the recipient server.
- Greylisting: Recipient-side greylisting may be temporarily rejecting unknown senders.
- IP Reputation: A new or poor IP reputation can lead to stricter rate limits and '451' errors.
Key considerations
- Sender Identification: Ensure proper sender identification (SPF, DKIM, DMARC) to improve deliverability.
- IP Warming: Gradually warm up new sending IPs to avoid triggering rate limits.
- Sending Volume: Monitor and adjust sending volume to stay within recipient server limits.
- Retrying Delivery: Since the error is temporary, retrying delivery at a later time is recommended.
Marketer view
Email marketer from Reddit shares that encountering '451' errors could also be linked to IP reputation. If the sending IP is relatively new or has a poor reputation, recipient servers might impose stricter rate limits.
23 Jun 2024 - Reddit
Marketer view
Email marketer from Email Marketing Forum explains the importance of warming up new sending IPs gradually. A sudden spike in email volume from a new IP address is more likely to trigger rate limits and '451' errors.
30 Sep 2021 - Email Marketing Forum
What the experts say
5 expert opinions
The 'Resources restricted - exceeds recipients limit' bounce message signifies that the sending rate or volume is perceived as too high by the receiving domain. This could involve temporary server issues, rate limiting, or resource constraints. For specific domains, such as 'optonline', significant sending limit reductions may be necessary. Accurately interpreting bounce messages requires investigation into sender reputation, content, and infrastructure, as there isn't a universal solution, and inaccurate reporting might indicate broader data reliability problems. 4xx errors suggest temporary deferrals, making retrying delivery a valid strategy.
Key opinions
- Rate Limiting: The sending rate is too high for the receiving domain's tolerance.
- Temporary Deferral: 4xx errors indicate temporary issues, suggesting a retry.
- Domain Specific Issues: Some domains, like 'optonline', may require drastic sending limit reductions.
- Data Reliability: Inaccurate delivery reporting can point to wider data integrity problems.
Key considerations
- Sender Reputation: Evaluate and improve sender reputation to reduce bounce occurrences.
- Sending Volume: Adjust sending volume to align with the recipient domain's limits.
- Retry Strategy: Implement a retry strategy for temporarily deferred messages.
- Bounce Interpretation: Accurately interpret bounce messages to identify and address underlying issues.
Expert view
Expert from Spam Resource advises that interpreting bounce messages accurately is crucial, but there is no one-size-fits-all solution. You need to investigate factors like sender reputation, content, and infrastructure.
17 Jun 2025 - Spam Resource
Expert view
Expert from Email Geeks interprets the bounce message "Resources restricted - exceeds recipients limit" as sending faster than the recipient domain thinks is acceptable.
15 Jul 2021 - Email Geeks
What the documentation says
4 technical articles
A '4.X.X' error, including the '451' error, as defined by RFC specifications, indicates a temporary failure in email delivery, suggesting the sender should retry. These errors often result from server-side throttling policies, like those used in Microsoft Exchange Server and Amazon SES, designed to manage resource utilization and prevent overload by enforcing sending limits. Specific SMTP enhanced status codes, such as '4.7.1', provide more granular detail about the cause, potentially pointing to relaying problems, access restrictions, or other security/policy breaches.
Key findings
- Temporary Failure: '4.X.X' errors are defined as temporary failures, warranting a retry attempt.
- Throttling Policies: Exchange Server and Amazon SES utilize throttling policies to manage resources and prevent server overload.
- Sending Limits: Exceeding sending limits can trigger '451' errors.
- Specific Status Codes: '4.7.1' indicates issues relating to relaying, access denial, or other security/policy violations.
Key considerations
- Retry Mechanism: Implement a retry mechanism for emails that initially result in temporary failure errors.
- Resource Management: Understand and respect sending limits imposed by recipient servers and email service providers.
- Policy Compliance: Ensure compliance with security and policy requirements to prevent access denial errors.
- Error Code Analysis: Analyze specific SMTP enhanced status codes for detailed information about the delivery problem.
Technical article
Documentation from IETF outlines how SMTP enhanced status codes, like '4.7.1', provide more specific information about delivery problems. A '4.7.1' code typically refers to issues related to relaying, access denied, or other security/policy violations.
11 Feb 2023 - IETF
Technical article
Documentation from RFC Editor defines a '4.X.X' error as a temporary failure. The sender is encouraged to try again later, as the condition might be transient.
6 May 2024 - RFC Editor
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