Email interface should be declared in a file called

A common convention!

In many programming languages and frameworks, it's common to declare an email interface in a file called IEmailInterface or EmailInterface. This follows the naming conventions of many programming languages, such as:

This naming convention is often used to indicate that the file contains an interface or abstract class that defines the contract or blueprint for an email service.

For example, in Java, you might have a file called IEmailInterface.java that contains the interface definition:

public interface IEmailInterface {
    void sendEmail(String to, String subject, String body);
}

Similarly, in C#, you might have a file called EmailInterface.cs that contains the interface definition:

public interface IEmailInterface
{
    void SendEmail(string to, string subject, string body);
}

By following this convention, it's easy to identify the file as containing an email interface, and developers can quickly find the interface definition when they need it.