Every valid email address must contain which symbol?

The correct answer is @ (at sign). Every email address follows the format local-part@domain, and the @ symbol is the essential separator that makes it valid.

Anatomy of an Email Address

• The @ Symbol: The \"at\" sign separates the user name (local part) from the domain name (host part). For example, in \"john.doe@example.com\", the @ divides \"john.doe\" and \"example.com\". Without it, the email system cannot route the message.
• Email Standard (RFC 5322): Internet standards require exactly one @ character in a valid email address. It cannot be omitted.
• Why Other Symbols Are Not Required:
  • #: Sometimes used in web URLs (fragment identifiers), not in emails.
  • $: Not mandatory; some special addresses might use it, but not required.
  • &: Not used in the standard email format.

Hence, every valid email address must contain the @ symbol.