cannot be transmitted through 7-bit data channels directly. To work around this, binary-to-text encodings have been devised which use only 7-bit ASCII characters. Some of these encodings are uuencoding, Ascii85, SREC, BinHex, kermit and MIME's Base64. EBCDIC-based systems cannot handle all characters used in UUencoded data. However, the base64 encoding does not have this problem.
Many early communications protocol standards, such as , , for SMTP, for NNTP, , , , were designed to work over such "7-bit" communication links. They specifically mention the use of ASCII character set "transmitted as an 8-bit byte with the high-order bit cleared to zero" and some of these: Appendix A, : 4.5.2., : Appendix B, : 4. explicitly restrict all data to 7-bit characters.
According to , the original definition of SMTP limits Internet Mail to lines (1000 characters or less) of 7-bit US-ASCII characters.: "SMTP as defined in limits the sending of Internet Mail to US-ASCII characters." Dan Sugalski. "E-mail with Attachments". "The Perl Journal". Summer 1999. "When mail was standardized way back in 1982 with RFC822, ... The only limits placed on the body were the character set (7-bit ASCII) and the maximum line length (1000 characters)."
Later the format of email messages was re-defined in order to support messages that are not entirely US-ASCII text (text messages in character sets other than US-ASCII, and non-text messages, such as audio and images). "Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages"
The Internet community generally adds features by "extension", allowing communication in both directions between upgraded machines and not-yet-upgraded machines, rather than declaring formerly standards-compliant legacy software to be "broken" and insisting that all software worldwide be upgraded to the latest standard. In the mid-1990s, people objected to "just send 8 bits (to SMTP servers)", perhaps because of a perception that "just send 8 bits" is an implicit declaration that ISO 8859-1 become the new "standard encoding", forcing everyone in the world to use the same character set. Instead, the recommended way to take advantage of 8-bit-clean links between machines is to use the ESMTP () 8BITMIME extension. Despite this, some Mail Transfer Agents, notably Exim and qmail, relay mail to servers that do not advertise 8BITMIME without performing the conversion to 7-bit MIME (typically quoted-printable, "Q-P conversion") required by . This "just-send-8" attitude does not in fact cause problems in practice, since virtually all modern email servers are 8-bit clean.