The checkmark in any email provider meant that the sender has a TLS certificate signed by a trusted CA.
The reason this exists is using cryptographic signing and public keys is to verify that the message is genuine and was not tampered with, as the only way to generate a valid email with a checkmark is to sign it with the corrisponding private key, making faking emails impossible.
Their certificate likely expired and they will have to renew it with their Certificate Authority.
Thank you for the report! We’ve resolved this issue with the verification checkmarks. You should see the emblem appearing correctly on all official emails.