The attack on Postmark's third-party data center resulted in a massive DDoS assault that lasted for approximately 44.5 hours, causing significant delays and disruptions to email delivery services for the company's customers. The attack, characterized as a UDP Reflection attack, generated hundreds of gigabits per second of traffic and was unusually long-lasting, lasting over four times longer than the average duration of similar attacks. Postmark's sending capacity was reduced by nearly 60x due to the mitigation efforts, causing mail queues to gather millions of messages. The company worked around the clock to mitigate the attack and redirect queued emails through alternate pathways, allowing some mail to flow while others experienced delays in events on activity pages. Despite the challenges, Postmark's team made adjustments to their sending volumes and communicated with receivers to avoid reputation and deliverability issues. Ultimately, email delivery returned to normal by 9:00am EST on December 17th, and the company has taken steps to improve its resilience and reliability against similar attacks in the future.