Why are my customer.io emails not delivering when using sendgrid?
Michael Ko
Co-founder & CEO, Suped
Published 2 Aug 2025
Updated 19 Aug 2025
7 min read
It can be incredibly frustrating to find that your emails aren't delivering, especially when you're using robust platforms like Customer.io alongside a reliable email service provider (ESP) such as SendGrid. You've set everything up, the platforms report messages as sent, yet your customers aren't receiving them. This is a common pain point for many businesses, and it often points to underlying issues beyond a simple 'sent' status.
When Customer.io and SendGrid are integrated, their combined power should lead to high deliverability. However, the complexity of this setup means there are multiple points where things can go awry. It's not always about whether the email left your system, but whether it successfully reached the recipient's inbox, or if it was filtered to spam or rejected outright.
Many factors can influence email delivery, ranging from technical configurations to sender reputation and content. Pinpointing the exact cause requires a systematic approach to troubleshooting, looking at the data from both platforms, and understanding the nuances of how email systems interact.
Initial checks and integration setup
When using Customer.io with SendGrid, especially via custom SMTP, the initial step is to confirm that the integration between the two platforms is correctly configured. A misconfigured API key or SMTP relay settings can prevent emails from even leaving Customer.io or being properly handed off to SendGrid.
I always recommend checking the API keys and authentication settings first. Ensure that the SendGrid API key used in Customer.io has the necessary permissions to send emails. Any discrepancy here can lead to immediate failures that might not be clearly reported as a typical bounce. This is often an overlooked initial step when delivery issues arise.
Beyond the initial setup, you should cross-reference delivery data from both platforms. Customer.io provides message statuses indicating whether a message was attempted or failed to send from their side. SendGrid, on the other hand, offers a detailed activity feed that shows the lifecycle of each email, including delivered, bounced, or deferred statuses. Comparing these logs can help identify where the disconnect is occurring.
API key verification
Ensure the API key you're using in Customer.io for SendGrid has the correct permissions. Sometimes, an API key might be restricted to certain IP addresses or actions, preventing emails from being sent successfully.
If you're expecting delivery data back into Customer.io, ensure that SendGrid webhooks are properly configured to send events (like bounces, clicks, opens) to Customer.io. A missing or incorrect webhook can lead to a discrepancy in reporting between platforms.
Diagnosing common deliverability problems
Once you've confirmed your integration, the next step is to diagnose the common reasons why emails might not be reaching the inbox. Bounce reasons are a critical piece of information. SendGrid categorizes bounces, and understanding these can tell you a lot about the issue. Common bounce types include hard bounces (permanent failure, e.g., invalid email address) and soft bounces (temporary failure, e.g., mailbox full or server issues).
One common issue, particularly with transactional emails or during high-volume sends, is rate limiting. Receiving ISPs (like Gmail) can temporarily block or delay emails if too many are sent from a single IP address or domain within a short period. SendGrid's logs would show these as deferred or rate-limited. If you're seeing a 50% delivery rate specifically to Gmail, rate limiting is a strong suspect, especially if other domains are not affected.
Another significant factor is your sender reputation, both for your domain and the IP addresses SendGrid uses to send your emails. A low reputation can lead to emails being sent directly to the spam folder or being blocked by Microsoft Outlook and HotmailMicrosoft inboxes. Factors contributing to poor reputation include high bounce rates, spam complaints, sending to inactive users, or being listed on a blocklist (or blacklist). Regularly monitoring your domain and IP health is crucial for consistent deliverability.
Common problems
Misconfigured integration: incorrect API keys or SMTP settings between Customer.io and SendGrid.
Rate limiting: sending too many emails too quickly to a single ISP, especially if shared IPs are used (e.g., Gmail).
Low sender reputation: high bounce rates, spam complaints, or being on a blocklist (or blacklist).
Authentication failures: incorrect SPF, DKIM, or DMARC records.
Solutions
Review configurations: double-check Customer.io and SendGrid settings, API keys, and webhooks.
Monitor logs: analyze SendGrid's activity feed for bounce reasons and deferred messages.
Warm up IPs: gradually increase sending volume to new IPs or domains to build trust.
Implement DMARC: set up a DMARC policy with monitoring to enforce proper authentication and gain visibility into your email ecosystem.
Proactive strategies and advanced troubleshooting
To prevent delivery issues, a proactive approach to email deliverability is key. Regular monitoring of your SendGrid account's deliverability metrics, such as bounce rates, spam complaint rates, and open rates, can provide early warnings. High bounce rates, for instance, can indicate an aging list or invalid email addresses, which can negatively impact your sender reputation. Similarly, consistently high spam complaints tell ISPs that your content might not be what recipients expect, leading to filtering.
Consider implementing a clear process for managing bounces and unsubscribes in Customer.io. Suppressing bounced addresses and respecting unsubscribe requests immediately is critical. Ignoring these can significantly harm your reputation and lead to your domain or IP being added to a blocklist (or blacklist), making it even harder to reach the inbox.
For advanced troubleshooting, especially when dealing with specific ISPs like Gmail, leverage Google Postmaster Tools. This free tool provides insights into your sending reputation, spam rates, and delivery errors specifically for Gmail recipients. Similar tools exist for Microsoft Outlook and HotmailMicrosoft and Yahoo Mail. These tools can offer detailed diagnostics that are not always available directly through your ESP.
DNS records for email authentication
SPF Record ExampleDNS
v=spf1 include:_spf.sendgrid.net ~all
Ensure your DNS records for SPF, DKIM, and DMARC are correctly set up and align with SendGrid's recommendations. An incorrect record can lead to emails being marked as spam or rejected by recipient servers.
Content and engagement
Beyond technical settings, the content of your emails and recipient engagement play a huge role. Avoid spammy keywords, broken links (e.g., SendGrid broken links), or excessive images. Focus on delivering relevant content to engaged subscribers to maintain a good sender reputation.
Sustaining good deliverability
Even with everything configured correctly, deliverability can still be a challenge. It's often about understanding the subtle signals ISPs look for and continuously optimizing your sending practices. From my experience, what works today might need adjustments tomorrow, given the dynamic nature of email security and filtering algorithms. Remaining agile and responsive to deliverability trends is critical.
One area often overlooked is list hygiene. Sending to invalid or disengaged email addresses not only wastes resources but also hurts your reputation. Regular list cleaning and implementing double opt-in processes can significantly improve your deliverability rates. This ensures you are only sending to people who genuinely want to receive your emails, which in turn leads to higher engagement and better inbox placement.
Finally, remember that deliverability is an ongoing process. It's not a set-it-and-forget-it task. Consistent monitoring, analysis of performance data, and prompt action on any red flags are necessary to maintain high inbox placement. Building and maintaining a strong sender reputation takes time and effort, but the rewards are well worth it in terms of reaching your audience effectively.
Views from the trenches
Best practices
Always cross-reference delivery data from Customer.io and SendGrid to pinpoint discrepancies.
Regularly monitor your SendGrid activity feed and suppression lists for insights into bounces and blocks.
Ensure your sending domain is properly authenticated with SPF, DKIM, and DMARC.
Common pitfalls
Overlooking API key permissions or incorrect SMTP settings in Customer.io's SendGrid integration.
Ignoring bounce reasons and deferrals, which can hide underlying reputation issues.
Failing to configure SendGrid webhooks to receive delivery event data back in Customer.io.
Expert tips
Leverage Google Postmaster Tools for detailed Gmail-specific deliverability insights.
Set up robust DMARC monitoring to get comprehensive reports on email authentication status.
Analyze engagement metrics (opens, clicks) to understand content reception and adjust strategy.
Expert view
Expert from Email Geeks says checking SMTP logs from SendGrid can provide clearer insights when Customer.io reports are vague. Additionally, reviewing suppression, bounce, and block lists within SendGrid is crucial for understanding delivery failures.
2021-12-27 - Email Geeks
Expert view
Expert from Email Geeks says if you are using SendGrid via Custom SMTP with Customer.io, reaching out to Customer.io's support team is advisable for a direct look into your specific setup.
2021-12-28 - Email Geeks
Achieving consistent email delivery
Resolving email delivery issues when using Customer.io with SendGrid involves a multi-faceted approach. It starts with meticulous verification of your integration settings, extends to understanding bounce reasons and potential rate limiting, and crucially, involves maintaining a strong sender reputation through proper authentication and list hygiene.
By leveraging the detailed logs and suppression lists available in SendGrid, alongside Customer.io's reporting, you can diagnose where emails are failing. Proactive measures such as implementing DMARC, monitoring Postmaster Tools, and continuously cleaning your email lists are essential for sustained deliverability.
Ultimately, a holistic view of your email program, from technical setup to content strategy and recipient engagement, is paramount. Addressing these areas systematically will help ensure your critical communications reach your customers' inboxes reliably, preventing the frustration of messages disappearing into the void.
Gmail) can temporarily block or delay emails if too many are sent from a single IP address or domain within a short period. SendGrid's logs would show these as deferred or rate-limited. If you're seeing a 50% delivery rate specifically to Gmail, rate limiting is a strong suspect, especially if other domains are not affected.
Microsoft Outlook and HotmailMicrosoft inboxes. Factors contributing to poor reputation include high bounce rates, spam complaints, sending to inactive users, or being listed on a blocklist (or blacklist). Regularly monitoring your domain and IP health is crucial for consistent deliverability.
