The Postmark SMTP servers experienced an outage on September 15-16, 2024, affecting some customers' ability to send emails. The SSL certificates for the servers expired, causing a disruption in service from midnight GMT until 9:40 UTC. An initial misdiagnosis by an engineer delayed the resolution of the issue, but another team responded promptly and updated the certificates, restoring functionality at approximately 9:16 UTC. To prevent similar incidents in the future, Postmark is implementing measures such as regular certificate audits, updating escalation paths within their engineering team, and enhancing internal communication channels to keep customers informed about any issues that may arise.