Simple Mail Transfer Protocol (SMTP) is a TCP/IP protocol used in sending and receiving e-mail. Yet user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For receiving messages, client applications usually use either POP3 or IMAP. To send a text message via the SMTP method, all one must do is attach their phone number to the beginning of the carriers SMTP email address and send it just like any other email. Proprietary systems such as Microsoft Exchange and Lotus Notes/Domino and webmail systems such as Hotmail, Gmail and Yahoo! Mail user their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.