<P> As this protocol started out purely ASCII text - based, it did not deal well with binary files, or characters in many non-English languages . Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP . Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8 - bit - clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8 - bit ASCII - like character encoding) via SMTP . Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII. 8 - bit - clean MTAs today tend to support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text . Recently the SMTPUTF8 extension was created to support UTF - 8 text, allowing international content and addresses in non-Latin scripts like Cyrillic or Chinese . </P> <P> Many people contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore . </P> <P> Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587 . Most mailbox providers still allow submission on traditional port 25 . The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine . Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host . Each process is an MTA (an SMTP server) in its own right . </P> <P> The boundary MTA uses the Domain name system (DNS) to look up the mail exchanger record (MX record) for the recipient's domain (the part of the email address on the right of @). The MX record contains the name of the target host . Based on the target host and other factors, the MTA selects an exchange server: see the article MX record . The MTA connects to the exchange server as an SMTP client . </P>

Which protocol is used to send email messages from a mail client to a mail server