Summary
Customer.io emails not delivering through SendGrid can stem from a multitude of factors. These include integration configuration issues (API keys, webhooks), authentication failures (SPF, DKIM, DMARC), poor sender reputation, rate limiting, blacklisted IPs, email content triggering spam filters, and inadequate list hygiene practices. Analyzing bounce reasons, checking suppression lists, warming up IP addresses, reviewing DNS settings, and potentially seeking direct support from Customer.io are recommended troubleshooting steps.
Key findings
- Integration Problems: Misconfigured Customer.io-SendGrid integration settings, like incorrect API keys or webhooks, can cause delivery failures.
- Authentication Issues: Failing to properly authenticate your domain (SPF, DKIM, DMARC) is a major contributor to deliverability problems.
- Reputation Damage: Poor sender reputation, often stemming from spam complaints or sending to invalid addresses, negatively impacts delivery rates.
- Rate Limits Exceeded: Hitting SendGrid's rate limits can cause emails to be throttled or blocked.
- Blacklisted IPs: Your IP address might be blacklisted due to past spam activity, hindering deliverability.
- Content Triggers: Email content containing spammy keywords, URLs, or attachments can trigger spam filters.
- List Hygiene Neglect: Sending emails to unengaged or invalid addresses damages your sender reputation and reduces deliverability.
- Infrastructure: Shared IP addresses may have poor reputations, and dedicated IP addresses need to be properly warmed up. Also ensure that reverse DNS is set up correctly.
- Gmail Rules: Gmail has strict authentication rules, that must be adhered to. Check the settings to ensure emails are being delivered
Key considerations
- Examine Logs & Suppressions: Regularly check SendGrid's SMTP logs, activity feed, and suppression lists to identify and address delivery issues.
- Verify Authentication: Ensure SPF, DKIM, and DMARC records are correctly configured for your sending domain.
- Warm Up IPs: If using a dedicated IP, gradually increase sending volume to establish a good reputation.
- Improve List Hygiene: Remove unengaged subscribers and validate email addresses to maintain a healthy list.
- Optimize Email Content: Review email content for spam triggers and follow email marketing best practices.
- Monitor Bounce Codes: Implement a system to monitor and analyze bounce codes to identify specific issues causing emails to fail.
- Check DNS Settings: Verify that DNS settings are correctly configured to ensure proper email authentication.
- Review Gmail Authentication: Review authentication for Gmail to see what is needed to be compliant.
- Seek Expert Assistance: Consider reaching out to Customer.io or SendGrid support for help troubleshooting complex delivery problems.
What email marketers say
8 marketer opinions
Several factors can cause Customer.io emails to fail delivery when using SendGrid. These include issues with SendGrid configuration (API keys, webhooks, event tracking), authentication problems (SPF, DKIM, DMARC), poor sender reputation, rate limiting, blacklisted IP addresses, and general list hygiene issues. SMTP logs and suppression lists should be checked, and Gmail's authentication rules should be followed.
Key opinions
- Configuration Issues: Incorrect setup of the Customer.io/SendGrid integration is a common problem. Verify API keys, webhooks, and event tracking settings.
- Authentication: Lack of proper authentication (SPF, DKIM, DMARC) can cause deliverability issues. Ensure these are correctly configured for your sending domain.
- Rate Limiting: SendGrid rate limits might be exceeded. Check your account limits and adjust sending practices accordingly.
- IP Blacklisting: Your sending IP address could be blacklisted due to previous spam or poor sending behavior. Investigate and take steps to remove it from blacklists.
- Suppression Lists: Check SendGrid's suppression lists (bounces, blocks) to identify recipients who are not receiving emails and understand why.
- Gmail Rules: Gmail has strict authentication rules, that must be adhered to. Check the settings to ensure emails are being delivered
Key considerations
- Monitor Logs: Regularly monitor SendGrid's SMTP logs and activity feed to identify delivery issues and diagnose their root causes.
- Improve Reputation: Maintain a good sender reputation by practicing good list hygiene (removing inactive subscribers, avoiding spam traps), and sending relevant content.
- Seek Support: Engage Customer.io support to investigate integration issues and get personalized assistance with deliverability troubleshooting.
- Review Gmail Authentication: Review authentication for Gmail to see what is needed to be compliant.
Marketer view
Marketer from Email Geeks (Deliverability Services Manager at Customer.io) offers direct support to investigate the Sendgrid/Customer.io integration issue, recommending the user contact the support team.
27 Apr 2022 - Email Geeks
Marketer view
Email marketer from Mailgun recommends verifying domain authentication settings (SPF, DKIM, DMARC) and monitoring the sender reputation. It also recommends checking for any blocks or restrictions.
19 Apr 2025 - Mailgun
What the experts say
3 expert opinions
Troubleshooting Customer.io emails failing to deliver through SendGrid involves examining bounce reasons, optimizing sending infrastructure (IP reputation, IP warm-up, reverse DNS), and improving list hygiene by focusing on opt-in contacts and removing inactive subscribers.
Key opinions
- Bounce Analysis: Understanding bounce reasons is crucial for diagnosing the cause of delivery failures.
- Infrastructure: Sending infrastructure issues, such as poor IP reputation or improper IP warm-up, can negatively impact deliverability. Reverse DNS configuration is also critical.
- List Quality: Poor list hygiene practices, including acquiring email lists, can damage sender reputation and affect deliverability.
Key considerations
- Investigate Bounces: Access and analyze bounce reasons to identify specific issues causing emails to fail.
- Optimize Infrastructure: Ensure shared IPs have a good reputation or properly warm up dedicated IPs. Configure reverse DNS correctly.
- Improve List Hygiene: Focus on obtaining opt-in contacts and regularly remove inactive subscribers to maintain a healthy email list.
Expert view
Expert from Spam Resource explains that problems can be related to your sending infrastructure. Shared IP addresses may have poor reputations, and dedicated IP addresses need to be properly warmed up. Also ensure that reverse DNS is set up correctly.
11 Nov 2023 - Spam Resource
Expert view
Expert from Email Geeks asks if the bounce reasons are accessible and what they indicate, to help troubleshoot the email delivery problem.
6 Feb 2024 - Email Geeks
What the documentation says
4 technical articles
Email delivery problems with Customer.io and SendGrid can arise from several technical factors. Incorrect DNS settings, poor sender reputation, high bounce rates, and spam complaints are common culprits. It's vital to warm up the IP address before sending emails to avoid being marked as spam. The content of the email itself, including URLs and keywords, can also trigger spam filters. Understanding SMTP bounce codes is critical for diagnosing delivery failures, as the server must provide a notification message explaining the reason for non-delivery.
Key findings
- DNS & Reputation: Incorrect DNS settings and a poor sender reputation are primary causes of delivery issues.
- IP Warm-up: Failure to properly warm up the IP address can lead to deliverability issues.
- Content Filters: Email content, including URLs and keywords, can trigger spam filters.
- Bounce Codes: SMTP servers provide bounce codes for undelivered messages. Understanding these codes is crucial for troubleshooting.
Key considerations
- Check DNS Settings: Verify that DNS settings are correctly configured to ensure proper email authentication.
- Warm Up IP: Gradually increase email volume to warm up the IP address and establish a good sending reputation.
- Review Content: Scrutinize email content to avoid spam triggers and ensure compliance with best practices.
- Analyze Bounce Codes: Implement a system to monitor and analyze bounce codes to identify and address delivery issues promptly.
Technical article
Documentation from Customer.io explains that the IP address should be warm before any emails are sent out. Otherwise the deliverability score can be affected and emails sent to spam.
22 Feb 2023 - Customer.io
Technical article
Documentation from RFC explains that if the SMTP server cannot deliver the message for some reason, it MUST construct and mail a notification message. It is important to understand these bounce codes to diagnose issues.
5 Feb 2025 - RFC
Are Sendgrid MTA retries damaging my sender reputation?
Are there email sending issues with AWS?
How accurate are SNDS colors in reflecting email delivery performance?
How can I diagnose and resolve email deliverability issues when emails aren't reaching intended recipients?
How can I improve email deliverability for a client with a poor sender reputation and questionable email acquisition practices?
How can I improve email deliverability when emails are going to spam?
