Summary
The message "rate limit exceeded" in email sending indicates that you have attempted to send emails at a rate higher than the recipient's server or your email service provider (ESP) allows. This is a common mechanism used by mailbox providers and ESPs to manage server load, prevent abuse, and maintain email deliverability. While it can be a temporary deferral, consistent rate limiting can signal underlying issues with your sending practices or sender reputation, potentially leading to more severe blocks or even blacklisting (or blocklisting).
Key findings
- Definition: Rate limiting means you've sent too many emails within a specified timeframe, triggering a temporary block or deferral by the receiving server.
- Purpose: Mailbox providers use rate limits to protect their systems from being overwhelmed and to filter out potential spam. As PubNub states, it refers to exceeding a maximum threshold of requests.
- Impact: While often a temporary issue, persistent rate limiting can harm your sender reputation and lead to emails being rejected or sent to the spam folder.
- Causes: Common causes include sending too many emails to a single domain too quickly, sending to a large percentage of invalid or inactive addresses, or a sudden spike in sending volume from a new or previously low-volume IP.
- Distinction from spam: A rate limit deferral is not necessarily a spam block. However, as Twilio highlights, exceeding thresholds can lead to rejections and higher bounce rates.
Key considerations
- Monitor delivery reports: Regularly check your email delivery reports and logs for bounce messages indicating rate limits or throttling.
- Implement throttling: Work with your technical team to ensure your email sending system has proper throttling mechanisms in place to gradually send emails, especially to new domains or large lists.
- Warm up new IPs: If using new IPs, follow a structured IP warming schedule to build reputation gradually. Even shared IPs from an ESP benefit from consistent, non-burst sending.
- Review list hygiene: Clean your email lists regularly to remove inactive or invalid addresses, which can contribute to higher bounce rates and trigger rate limits.
- Understand ESP limits: Be aware of the sending limits and best practices of your chosen Email Service Provider (ESP), like Amazon SES, MailChimp, or GetResponse, as they have their own internal throttling rules.
What email marketers say
Email marketers often encounter "rate limit exceeded" messages and typically view them as a signal to review sending practices rather than an immediate disaster. They tend to attribute these issues to a combination of sending volume, frequency, and potentially underlying content quality issues, even if explicit spam percentages appear low. The general consensus among marketers is to prioritize thoughtful sending strategies and proper throttling.
Key opinions
- Simultaneous issues: Some marketers observe that rate limiting can occur alongside other issues like suspected spam, even if the spam rate is reported as 0%.
- Throttling oversight: A common cause for rate limits is sending emails too quickly or dumping a large volume in a short period, indicating a need for better throttling.
- Reputation concern: While a single instance of rate limiting might not severely damage reputation, marketers are wary of its long-term impact if not addressed.
- Overlap of sends: Sending campaigns from multiple ESPs (e.g., MailChimp, GetResponse, SES) concurrently can sometimes lead to rate limiting due to overlapping sends to the same recipient domains.
Key considerations
- Developer coordination: Marketers emphasize clear communication with web development teams to ensure proper custom throttling for large broadcasts, especially when using services like Amazon SES.
- Monitor shared IPs: When using shared IP addresses, be mindful of how your sending volume might interact with other users on the same IPs.
- Slow down sending: If rate limiting occurs, marketers often advise reducing the sending speed, even if it means extending the broadcast duration to several hours. IONOS documentation suggests this as a solution.
- Post-incident review: After a rate limit incident, it's crucial to review all sending configurations, including rDNS, EHLO/HELO values, and connection speeds to prevent recurrence. This helps boost email deliverability rates.
Marketer view
Email marketer from Email Geeks indicates that Google might be flagging both excessive sending speed and spam at the same time. This suggests a dual concern from the receiving server, where volume and content quality are simultaneously under scrutiny.
Marketer view
Email marketer from Email Geeks suggests that a 0% display for suspected spam, despite a rate limit, could be a reporting bug. It is advisable to assume that spam flagging still occurred and act accordingly to address potential content or list issues.
What the experts say
Email deliverability experts generally agree that "rate limit exceeded" is a sign to investigate, but not necessarily a cause for panic, especially if it's an isolated event. They emphasize that while some rate limits are purely volume-based, they can also indirectly indicate mailstream cleanliness or reputation issues. Experts advocate for meticulous configuration, a deep understanding of recipient policies, and a cautious approach to sending volume and frequency to maintain good deliverability.
Key opinions
- Recipient throttling: Experts note that recipient throttling, while not always linked to spam, can indicate that the mailstream is not as clean as the ISP desires.
- Immediate blocking: Sometimes, a rate limit simply means the recipient's mail exchange (MX) server has said "no" to a rapid dump of emails.
- Low deferral isn't bad: A low deferral rate, such as 1%, is often considered acceptable and not a major concern by some experts.
- Reputation is dynamic: While a single rate limit event can affect reputation, it's measured over time. Many good sending days can counteract a brief spike in issues. For more, see our guide on recovering domain reputation.
- Gmail specifics: One expert notes that a direct "rate limit exceeded" tempfail for reputation reasons is rare at Gmail; instead, it often indicates an individual recipient being overwhelmed, possibly due to mailbombing, as outlined in Google's receiving rate documentation.
Key considerations
- Configuration review: Carefully examine your sending setup, including rDNS, EHLO/HELO values, and the number of connections. This is fundamental to avoiding rate and connection limits.
- New IPs vs. established: The impact of a rate limit can differ between new and established IPs. New IPs require a careful warm-up, while established ones might experience temporary issues due to sudden volume changes.
- One-off vs. recurring: If it's a one-time incident, monitoring the next send is sufficient. If it recurs, a deeper investigation into sending patterns and infrastructure is warranted. This proactive monitoring ties into effective deliverability testing.
- Susceptibility to attacks: Check if your forms are susceptible to automated sign-ups or mailbombing, as this can lead to legitimate recipients being rate-limited.
Expert view
Email expert from Email Geeks suggests that if the recipient is merely throttling, it may not be reported as suspected spam. However, exceeding a delivery rate throttle might still indicate that the mail stream is not as clean as the recipient ISP prefers.
Expert view
Email expert from Email Geeks explains that sometimes, a rate limit can simply mean the recipient's mail exchange server rejects a large burst of emails, effectively saying "no" to the sudden volume.
What the documentation says
Official documentation from various providers, including Microsoft, IONOS, and general email deliverability resources, consistently define "rate limit exceeded" as a mechanism to control email flow and prevent abuse. The documentation highlights that such limits are enforced to protect infrastructure and maintain service quality. Exceeding these limits typically results in temporary rejections or deferrals, with clear instructions on how senders should respond, primarily by reducing sending velocity.
Key findings
- System protection: Rate limits are in place to prevent systems from being overwhelmed by too many requests or emails in a short period, as highlighted by PubNub.
- Consequences of exceeding: Exceeding limits leads to blocks on further outgoing emails, resulting in non-delivery reports (bounces), as stated by Retarus regarding Microsoft.
- SMTP error codes: Common SMTP error codes like '421 Rate limit reached' are explicit indicators of too many emails sent too quickly, as shown in IONOS documentation.
- Throttling definition: Throttling is often described as limiting the maximum number of messages per minute that can be sent from a single source.
- ISP rejection: ISPs will reject emails sent above their acceptable thresholds, leading to higher bounce rates, as Twilio explains.
Key considerations
- Reduce sending speed: The primary solution recommended is to reduce the number of emails sent within a short timeframe.
- Temporary restrictions: Accounts that exceed limits may face temporary restrictions on sending additional emails.
- Monitor delivery errors: Closely watch for specific bounce messages, such as '450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered', from providers like Google.
- Provider-specific limits: Different email providers and services (e.g., Outlook, Gmail) have their own specific limits on daily emails or recipients, which must be respected.
- HTTP status codes: For API-related limits, the '429 Too Many Requests' HTTP status code is commonly used, indicating a rate limiting enforcement.
Technical article
Documentation from PubNub states that exceeding the limit refers to a situation where a system or service has enforced a maximum threshold on the rate of requests or actions. This mechanism helps maintain system stability and prevent overload.
Technical article
Documentation from Retarus Corporate Blog highlights that exceeding the Transient Email Rate Limit (TERRL) results in a block on further outgoing emails. Users will then receive non-delivery reports (bounces).
Related resources
6 resources
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