FWIW, I can't connect via my home or office fiber (separate providers), but on my T-Mobile 5G it works fine. When I do a traceroute it gets stuck in Cloudflare's network. I'm in Europe.
EDIT: I did some more testing from some other machines around the world. Most are able to connect, some aren't. When I compare the traceroutes it looks like a networking issue on Fastmail's end. They look mostly identical except for the last hop. That's also what their status page says[1].
[1] https://fastmailstatus.com/clji5sqvg99220wtohhj3e2d2b