New, tightened bulk sender rules by Gmail went into effect in November 2025, which affected Kustomer’s ability to deliver transactional emails to Gmail mailboxes. Specifically, DMARC alignment failures due to messages not being aligned to the from domain resulted in emails sent from mail.kustomerapp.com domain to gmail.com addresses to be blocked.
Root Cause
Beginning in early November 2025, Gmail rolled out stricter requirements for organizations that send large volumes of email. Because Kustomer sends more than 5,000 messages per day to Gmail addresses, Gmail classifies our traffic as “bulk sender” traffic and applies stronger authentication checks.
Many of the emails sent from Kustomer customers through Postmark were not fully meeting Gmail’s updated authentication requirements. Although the messages were being sent from addresses that looked like they belonged to each customer (eg: support@orgName.mail.kustomerapp.com), the DMARC policy check was now failing. As a result, Gmail could not verify that these messages truly came from the domain shown in the “From” address.
When Gmail applied its new enforcement rules, any email that did not clearly authenticate as coming from the correct domain was blocked before delivery. This caused a spike in rejected emails for customers sending to Gmail inboxes.
Early November 2025
Gmail begins enforcing newly updated bulk-sender requirements for high-volume senders. These rules require stricter domain authentication for all messages sent to Gmail inboxes.
Nov 11, 2025
2:18 PM EST — Initial ReportsCustomer support teams begin receiving reports that emails sent to Gmail addresses are bouncing or failing to deliver.
2:46 PM EST — Issue Confirmed
Responding teams identify that the issue is widespread and related to Gmail’s new enforcement behavior.
3:11 PM EST — Root Cause Narrowed DownMisalignment in required email authentication settings is identified as the likely cause of Gmail rejecting messages.
4:00 PM EST - 7:45 PM EST – Fix Validation
Responding engineers work on implementing and validating the fix.
8:01 PM EST — Fix deploymentThe necessary configuration updates are confirmed, including correcting DNS records so Gmail can properly verify the sender.
10:00 PM–11:00 PM EST — Remediation & StabilizationUpdated DNS records are applied across affected customer domains. Gmail begins accepting previously rejected messages, and email delivery to Gmail addresses returns to normal.
Lessons/Improvements
Improved Email Authentication Setup Across All Customers: All customer organizations now have the correct DNS records and Postmark sender signatures in place to ensure their messages meet Gmail’s authentication requirements.
Enhanced Monitoring for Email Delivery Issues: We are implementing new logging and alerting to detect spikes in email bounces earlier, allowing us to respond more quickly if delivery issues arise in the future.
Stronger Setup Process for New Integrations: Going forward, all new Postmark email integrations will be configured with the correct authentication settings from the start to prevent this issue from occurring.