What causes Gmail delays due to unusual sending rates and suspicious content?

9 Marketer opinions 7 Expert opinions 4 Technical articles 2 Resources

Summary

Gmail delays due to unusual sending rates and suspicious content are multifaceted, stemming from Gmail's spam prevention systems. At its core, an 'unusual rate' signifies a deviation from established sending patterns, triggering scrutiny. Suspicious content, characterized by spam trigger words, poor formatting, and excessive links, is actively filtered. A sender's reputation, built on consistent volume, low complaint rates, and proper authentication (SPF, DKIM, DMARC), is crucial; poor list hygiene and sending from multiple sources can negatively impact it. Technical aspects, like exceeding sending limits and encountering SMTP 421 or 4xx errors (indicating rate limiting or content filtering), also contribute. Mitigation strategies involve warming up IP addresses, optimizing content, maintaining list hygiene, properly authenticating emails, monitoring sender reputation using Google Postmaster Tools, checking campaign cadence and KPIs, and generally following best practices for bulk email senders.

Key findings

Unusual Sending Rates: Sudden spikes in sending volume trigger rate limiting and spam filters.
Suspicious Content: Spam trigger words, poor HTML, and excessive links flag emails as suspicious.
Sender Reputation Impact: Poor list hygiene, lack of authentication, and high complaint rates damage sender reputation.
Technical Errors: SMTP 421 and 4xx errors indicate temporary issues like rate limiting and content filtering.
Authentication Importance: Proper authentication (SPF, DKIM, DMARC) is crucial for establishing trust with Gmail.

Key considerations

IP Warming Strategies: Gradually warm up new IP addresses to establish a positive sending history.
Content Optimization: Use spam checkers and adhere to email design best practices to avoid triggering filters.
List Hygiene Maintenance: Regularly clean email lists to remove unengaged subscribers and improve deliverability.
Authentication Configuration: Configure SPF, DKIM, and DMARC records to authenticate email and improve deliverability.
Reputation Monitoring: Use Google Postmaster Tools to monitor sender reputation and identify potential issues.
Adhere to Sending Limits: Stay within Gmail's sending limits to avoid triggering delays or restrictions.
Review Analytics: Campaign Cadence and KPIs need to be reviewed so you are aware of the health of your sending.

What email marketers say
9Marketer opinions

Gmail delays due to unusual sending rates and suspicious content are primarily caused by Gmail's spam prevention mechanisms. These mechanisms include rate limiting, content filtering, and sender reputation assessment. Sudden spikes in sending volume, especially from new IP addresses or domains, trigger rate limits and require IP warming. Suspicious content, such as spam trigger words, poor HTML formatting, or excessive links, is flagged by content filters. Poor email list hygiene and improper email authentication (SPF, DKIM, DMARC) negatively impact sender reputation, leading to delays and filtering issues. Following email design best practices, cleaning email lists, warming up IP addresses, and using Google Postmaster Tools are crucial for avoiding these delays.

Key opinions

Sending Rates: Unusual sending rates, especially sudden spikes, trigger Gmail's rate limiting mechanisms.
Suspicious Content: Spam trigger words, poor HTML formatting, and excessive links are flagged by Gmail's content filters.
Sender Reputation: Poor email list hygiene and improper email authentication negatively impact sender reputation.
Authentication: Gmail needs to authenticate the senders identity, using SPF, DKIM, DMARC protocols. If these are not configured correctly then delays may occur.

Key considerations

IP Warming: Gradually increase sending volume over time to establish a positive sending history with ISPs.
Content Optimization: Use a spam checker and follow email design best practices to avoid triggering content filters.
List Hygiene: Regularly clean your email list to remove unengaged subscribers and improve sender reputation.
Authentication Protocols: Checking and configuring email authentication, which is important for being able to send to Gmail.

Marketer view

Email marketer from Litmus explains that improper authentication protocols (SPF, DKIM, DMARC) can cause messages to be delayed or blocked due to Gmail having no way to verify the senders identity.

August 2022 - Litmus

Marketer view

Email marketer from GMass Blog explains that the use of certain words, phrases, or content types (such as excessive exclamation points, or ALL CAPS) are more likely to trigger spam filters, cause delays, and negatively affect sender reputation.

May 2024 - GMass Blog

Marketer view

Email marketer from Mailjet Blog explains that suspicious content, such as spam trigger words, excessive use of links, or poor HTML formatting, can flag emails as spam and cause delays. They advise using a spam checker and following email design best practices.

October 2023 - Mailjet Blog

Marketer view

Email marketer from EmailToolTester explains that poor email list hygiene, such as sending to inactive or invalid email addresses, can negatively impact sender reputation and lead to delays. They recommend regularly cleaning your email list to remove unengaged subscribers.

November 2022 - EmailToolTester

Marketer view

Email marketer from Reddit shares that Gmail's algorithms are sensitive to sudden changes in sending behavior. If you suddenly start sending a lot more emails than usual, or if your content contains suspicious keywords or links, Gmail may delay your messages.

November 2023 - Reddit

Marketer view

Email marketer from StackOverflow explains that Gmail imposes rate limits to prevent abuse. Exceeding these limits can result in temporary delays or sending restrictions. The specific limits vary depending on the type of account and the sender's reputation.

October 2021 - StackOverflow

Marketer view

Email marketer from HubSpot explains that the practice of 'warming up' IP addresses will create a gradual trust with Gmail. As opposed to flooding their servers with high volume and suspicious content.

April 2024 - HubSpot

Marketer view

Email marketer from Email Geeks shares advice to check authentication protocols, domain reputation, content, HTML, links (SSL certificates), campaign cadence, and KPIs; also recommends using Google Postmaster Tools and reducing sending volume initially.

February 2024 - Email Geeks

Marketer view

Email marketer from SendGrid Blog shares that a sudden increase in sending volume from a new IP address can negatively impact sender reputation and lead to delays. They recommend gradually increasing sending volume over time (IP warming) to establish a positive sending history with ISPs.

September 2023 - SendGrid Blog

What the experts say
7Expert opinions

Gmail delays related to unusual sending rates and suspicious content stem from Gmail's efforts to combat spam. An 'unusual rate' directly signifies a deviation from established sending patterns, signaling potential issues. These problems often originate from problematic content within the emails themselves, as well as customer-related factors like new accounts or multiple sending sources. It is important to be cautious of click-tracking domains, which can negatively impact reputation. Gmail, like other ISPs, actively uses content filtering to identify and delay suspicious messages based on spam trigger words, formatting, and links. Maintaining a positive sender reputation through consistent volume, low complaint rates, and proper authentication is essential, and good list hygiene via regular cleaning is vital for avoiding deliverability issues.

Key opinions

Unusual Rate: Deviations from normal sending patterns trigger Gmail's spam detection.
Problematic Content: Suspicious elements within email content (trigger words, formatting, links) cause delays.
Sender Reputation: A poor sender reputation, from various factors, contributes to delivery delays.
Content Filtering: Gmail actively scans content for spam signals and suspicious URLs.

Key considerations

Monitor Sending Patterns: Maintain a regular sending volume to avoid triggering rate limits.
Review Email Content: Carefully review email content for spam trigger words and suspicious formatting.
Maintain List Hygiene: Regularly clean your email list to remove unengaged or invalid addresses.
Authenticate Email: Ensure proper email authentication (SPF, DKIM, DMARC) to establish trust with Gmail.
Monitor Reputation: Regularly monitor your sender reputation using Google Postmaster Tools.

Expert view

Expert from Email Geeks states that an unusual rate refers to exactly that: an unusual rate.

November 2024 - Email Geeks

Expert view

Expert from Email Geeks suggests the issues are due to an unusual rate of mail and potentially problematic content, possibly related to a new customer or a customer sending from multiple sources.

September 2024 - Email Geeks

Expert view

Expert from Word to the Wise highlights that one of the biggest problems, causing deliverability issues, is not correctly cleaning lists and maintaining good list hygiene. Regularly removing unengaged subscribers will help with sender reputation and delivery times to Gmail.

March 2022 - Word to the Wise

Expert view

Expert from Word to the Wise (Laura Atkins) explains that maintaining a good sender reputation is crucial for avoiding Gmail delays. She states that factors like consistent sending volume, low complaint rates, and proper authentication help build and maintain a positive reputation. Sending unusual amounts will negatively impact this.

February 2023 - Word to the Wise

Expert view

Expert from Email Geeks suggests that the content of the emails themselves is the problem, rather than an external source, based on the "your links" message.

May 2022 - Email Geeks

Expert view

Expert from Spam Resource (John Levine) explains that Gmail, like other ISPs, uses content filtering to identify and delay messages that appear suspicious. This includes looking for spam trigger words, unusual formatting, and suspicious links.

January 2025 - Spam Resource

Expert view

Expert from Email Geeks advises caution if using the same domain for click-tracking across all customers instead of white-labeled domains.

January 2023 - Email Geeks

What the documentation says
4Technical articles

Gmail delays due to unusual sending rates and suspicious content are primarily a result of Gmail's spam prevention measures. Google Workspace Admin Help documentation indicates that Gmail imposes sending limits to prevent spam and abuse, and exceeding these limits can lead to temporary delays. Google Postmaster Tools Help outlines best practices for bulk senders, emphasizing authentication, sender reputation, and low spam complaint rates. RFC 5321 defines SMTP 421 errors as temporary server issues, possibly due to rate limiting. SparkPost documentation details that 4xx SMTP response codes signify temporary errors like rate limiting or content filtering, suggesting a retry.

Key findings

Sending Limits: Gmail imposes sending limits to prevent spam and abuse, with exceeding these limits causing delays.
Best Practices: Authentication, maintaining sender reputation, and ensuring low spam complaint rates are crucial for bulk senders.
SMTP 421 Errors: SMTP 421 errors indicate temporary server issues potentially caused by rate limiting.
SMTP 4xx Errors: 4xx SMTP response codes signify temporary errors, including rate limiting or content filtering issues.

Key considerations

Adhere to Sending Limits: Monitor and adhere to Gmail's sending limits to avoid triggering delays.
Implement Best Practices: Implement authentication, maintain sender reputation, and aim for low spam complaint rates.
Handle Temporary Errors: Treat SMTP 421 errors as temporary and retry sending the message later.
Check Response Codes: If using a mail server, monitor 4xx response codes.

Technical article

Documentation from RFC 5321 defines the SMTP 421 error code as "Service not available, closing transmission channel." It indicates a temporary server issue, which can be caused by resource constraints or rate limiting. Retrying the message later may succeed.

January 2025 - RFC 5321

Technical article

Documentation from Google Workspace Admin Help explains that Gmail imposes sending limits to prevent spam and abuse. Exceeding these limits can result in temporary delays or sending restrictions. Unusual sending patterns, such as a sudden increase in volume, can trigger these limits.

June 2024 - Google Workspace Admin Help

Technical article

Documentation from SparkPost details the SMTP response codes that will be encountered during email delivery. Response codes starting with a 4xx indicate a temporary error, which may be caused by rate limiting, content filters, or temporary server issues. In the case of 421 or 450 response codes it's typically a good idea to wait and retry the delivery.

August 2023 - SparkPost

Technical article

Documentation from Google Postmaster Tools Help outlines best practices for bulk email senders, emphasizing the importance of authenticating email (SPF, DKIM, DMARC), maintaining a good sender reputation, and ensuring a low spam complaint rate. Failure to adhere to these guidelines can lead to delays and filtering issues.

April 2021 - Google Postmaster Tools Help