The available evidence (only being blocked in Chicago) would actually point to a technical issue as opposed to an intentional block.
What's special about Chicago?
We have our four core ZeroTier root servers on different providers at different data centers for this reason. Entire data centers or cloud regions becoming unreachable from a certain ISP in a certain area for a period of time is not all that uncommon.
In case you don't know: the core of the Internet is basically a ball of chewing gum, duct tape, glue, twine, and carrier pigeons maintained by grumbling BGP admins.
Sounds more like a routing misconfiguration that AT&T can't pin down.
Traceroutes to it and www.tutanota.com seem to die in similar or the same place.
Which both last end there currently. Last two not octets were different. Few minutes ago.
I don't know enough about ipv6 to diagnose further, or how to force my phone to ipv4 on lte.
We are tracking it here https://meta.discourse.org/t/cannot-reach-one-forum-on-at-t-...
BTW, if these tutanota guys were smart, they'd be pitching their services to AT&T instead of dragging them through the mud.
Last I checked, AT&T had subcontracted Yahoo as their mail handler. Nowhere to go but up from that mess...