निम्नलिखित में से कौन सा वर्ण (character) ईमेल आईडी का एक अनिवार्य हिस्सा होना चाहिए?

Which of the following is defined as a set of programs, standards and protocols that allow the text, images, animations, sounds and videos to be stored, accessed, and linked together in form of websites?

In the context of email management, what is the primary purpose of using search or filter tools inside an email account?