Look at
https://status.fastmail.com and you will see the following:
At 15:49:02 UTC:
We’re currently experiencing issues that will cause interrupted service. We do not expect any mail to be lost. We are currently investigating.
At 17:10:15 UTC:
Access to Fastmail services has been restored. System status is currently normal for all users. We're continuing to monitor the situation. Some users will experience residual delays with mail delivery. No mail has been lost.
Bill