when trying to send the 221 Bye reply. SMTP Extensions Extension discovery mechanism Clients learn a server's supported options by using the EHLO greeting, as exemplified below, instead of the original HELO. Clients fall back to HELO only if the server does not support EHLO greeting. Modern clients may use the ESMTP extension keyword SIZE to query the server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute. Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the HELO command with the EHLO command. S: 220 smtp2.example.com ESMTP Postfix C: EHLO bob.example.org S: 250-smtp2.example.com Hello bob.example.org [192.0.2.201] S: 250-SIZE 14680064 S: 250-PIPELINING S: 250 HELP Thus smtp2.example.com'' declares that it can accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). In the simplest case, an ESMTP server declares a maximum SIZE immediately after receiving an EHLO. According to , however, the numeric parameter to the SIZE extension in the EHLO response is optional. Clients may instead, when issuing a MAIL FROM command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages. Binary data transfer Original SMTP supports only a single body of ASCII text, therefore any binary data needs to be encoded as text into that body of the message before transfer, and then decoded by the recipient. Binary-to-text encodings, such as uuencode and BinHex were typically used. The 8BITMIME command was developed to address this. It was standardized in 1994 as It facilitates the transparent exchange of e-mail messages containing octets outside the seven-bit ASCII character set by encoding them as MIME content parts, typically encoded with Base64. Mail delivery mechanism extensions On-Demand Mail Relay On-Demand Mail Relay (ODMR) is an SMTP extension standardized in that allows an intermittently-connected SMTP server to receive email queued for it when it is connected. Internationalization extension Original SMTP supports email addresses composed of ASCII characters only, which is inconvenient for users whose native script is not Latin based, or who use diacritic not in the ASCII character set. This limitation was alleviated via extensions enabling UTF-8 in address names. introduced experimental UTF8SMTP command and later was