In Zendesk Support, the Ticket Received At condition in triggers, automations, and views can check addresses from the Zendesk Support email domain as well as from external email domains. The external email domain must be set up as described in Using an external email domain or the condition won't work.
Before March 15, 2013, the condition only supported addresses from the Zendesk Support email domain. As a result, you didn't have to specify "zendesk.com" in the address field because it was assumed.
You don't need to do anything if you added the condition in any of your business rules before that date. However, if you make a change to one of those rules in the future, you'll need to update the value of the Ticket Received At condition to include the rest of your email address (@mycompany.zendesk.com).