Are Email-Ids Case sensitive?

Are Email-Ids Case sensitive?

Looks like email-ids are sensitive. I have a business email with Zoho.

I tested it multiple times and emails to id with name accurately matching, works but for ex:  if i change first alphabet in the email to upper case, the emails will bounce with eror: "Domain not resolved". This not the case in other email providers. kindly assist as we might miss emails from customers / client.