Beginning 2:36 AM US Pacific time, Mailsac internal monitoring indicated slowness due to an abnormally large amount of spam coming from China. By approximately 6:30 AM we identified all root causes and believe the issue is resolved.
Our service employs several methods of blocking, shaping, and throttling egregious traffic from unpaid users. This particular attack worked around these automatic mitigation efforts, in part because the attackers opened thousands of sockets and left them open a long time, exploiting a loophole in our SMTP inbound receiver code.
Here is a graph of our inbound message rate showing the attack compared to baseline.