Back in 2019, UPC (Liberty Global) sold their Czech Republic holdings to Vodafone. Fast forward to May 2020 and they've just announced that email service to the Czech UPC/Chello domains is being shut down, with service terminating on August 31, 2020.
The affected email domains are: upcmail.cz, chello.cz, karneval.cz, mistral.cz and mbox.dkm.cz.

[ H/T: Mailkit's Jakub Olexa ]