Over the last two days, service has been interrupted several times. This is the result of an ongoing attack against Fastmail as well as other email providers. We're working with network service providers and law enforcement to put an end to the problem.
Fastmail users and interested parties, you can find current status information on their system status site here. Their two most recent updates that I see (current as of 6:25 pm Chicago time on Saturday October 23rd) combine to say:
"We have multiple mitigations in place against the DDOS attacks and we are continuing to monitor. No mail has been lost, your data remains safe and services are operational." ... "DDOS protection can throttle or disrupt customer traffic. Some regions are affected if they are where attack traffic is also coming from. We apologize to affected customers."
Currently, from Chicago, I am able to access my Fastmail account successfully, and I'm able to send and receive mail there. Hopefully that means their DDOS-mitigation efforts are working and that they're recovering.
October 25, 2021 Update: Things seem to have returned to normal. Fastmail is reporting the following on their status page: "System status is currently normal for all users. We're continuing to monitor the situation for any further hostile activity. No mail has been lost throughout this period."
[ H/T: Jennifer Nespola Lantz ]