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.
The provider is encouraging all users to move to Gmail or Seznam for future email service. There does not appear to be any sort of automated transition process, so it would not be possible for senders to simply change a recipient's domain name to keep their email address valid. Senders will want to ask subscribers for an updated email address, sometime before the shutdown at the end of August.
[ H/T: Mailkit's Jakub Olexa ]