superseded by that introduced SMTPUTF8 command. These extensions provide support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as Greek and Chinese. Current support is limited, but there is strong interest in broad adoption of and the related RFCs in countries like China that have a large user base where Latin (ASCII) is a foreign script. Extensions Like SMTP, ESMTP is a protocol used to transport Internet mail. It is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol. The main identification feature for ESMTP clients is to open a transmission with the command EHLO (Extended HELLO), rather than HELO (Hello, the original standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821 compliant server returns error code 500, allowing ESMTP clients to try either HELO or QUIT. Each service extension is defined in an approved format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA). The first definitions were the RFC 821 optional services: SEND, SOML (Send or Mail), SAML (Send and Mail), EXPN, HELP, and TURN. The format of additional SMTP verbs was set and for new parameters in MAIL and RCPT. Some relatively common keywords (not all of them corresponding to commands) used today are: 8BITMIME – 8 bit data transmission, ATRN – Authenticated TURN for On-Demand Mail Relay, AUTH – Authenticated SMTP, CHUNKING – Chunking, DSN – Delivery status notification, (See Variable envelope return path) ETRN – Extended version of remote message queue starting command TURN, HELP – Supply helpful information, PIPELINING – Command pipelining, SIZE – Message size declaration, STARTTLS – Transport Layer Security, (2002) SMTPUTF8 – Allow UTF-8 encoding in mailbox names and header fields, UTF8SMTP – Allow UTF-8 encoding in mailbox names and header fields, (deprecated) The ESMTP format was restated in (superseding RFC 821) and updated to the latest definition in in 2008. Support for the EHLO command in servers became mandatory, and HELO designated a required fallback. Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an EHLO message keyword starting with "X",