On Thursday, January 22, 2026, between 13:14 and 15:04 UTC, a new change in our background job processing caused manually triggered invoice emails to be treated as repeatable jobs instead of one‑time actions. This led to the same invoice emails and related print jobs being sent repeatedly to about 400 recipients, mainly in hotels in France, DACH, and Sweden.
The resulting spike in outbound emails later caused email providers to defer or block part of our traffic, about 2% of all emails being sent, including some login emails.
We reverted the deployment to the previous stable version to prevent further impact on customers.
For blocked emails, we worked with our email service provider to remove any affected addresses from blocklists.
For blocked or delayed logins, our support team used temporary workarounds so that the impacted users could regain access while deliverability stabilised.
We monitored the email delivery over the weekend (Friday, Jan 23 - Sunday, Jan 25) for any further degradation.
A change to our background processing logic meant one-time background tasks, like sending an invoice email, were not correctly marked as finished. Instead of running once and stopping, these tasks were put back into the queue and run again, repeatedly sending the manually triggered invoice emails.
The large increase in email volume (about 2x), including many repeated messages to the same recipients, caused some providers to treat our traffic as spam‑like, which led to a spike in deferred and blocked emails, including authentication emails.
Strengthen automated checks around how background tasks are not only run, but also cleaned up.
Improve detection by adding alerting for unusual email volumes (overall and per recipient), abnormal task retry patterns, and spikes in deferred or bounced emails, so we can react much earlier.
To both improve detection and reduce future impact, we are exploring options for rate‑limiting emails sent to a single recipient and separating login‑related emails into a dedicated flow that is less affected by issues with other email types.