what is the part of the email after the @ called
Email address An email address such as John.Smith@example.com is made up of a local-part, an @ symbol, then a case-insensitive domain. Although the standard specifies the local part to be case-sensitive, in practice the mail system at example.com may treat John.Smith as equivalent to JohnSmith or even as johnsmith,[1] and mail systems often limit their users' choice of name to a subset of the technically valid characters. In some cases they also limit which addresses it is possible to send mail to.
Simple Mail Transfer Protocol Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.