SMTP and NNTP 8-bit Cleanness
Historically, various media were used to transfer messages, some of them only supporting 7-bit data, so an 8-bit message had high chances to be garbled during transmission in the 20th century. But some Internet implementations really did not care about formal discouraging of the 8-bit data and allowed high bit set bytes to pass through.
Many early communications protocol standards, such as RFC 780, RFC 788, RFC 821 for SMTP, RFC 977 for NNTP, RFC 1056, RFC 2821, RFC 5321, were designed to work over such "7-bit" communication links. They specifically mention the use of ASCII character set "transmitted as a 8-bit byte with the high-order bit cleared to zero" and some of these explicitly restrict all data to 7-bit characters.
For the first few decades of email networks (1971 to the early 1990s), most email messages were plain text in the 7-bit US-ASCII character set.
According to RFC 1428, the original RFC 821 definition of SMTP limits Internet Mail to lines (1000 characters or less) of 7-bit US-ASCII 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).
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 world-wide be upgraded to the latest standard. In the mid-1990s, people objected to "just send 8 bits (to RFC 821 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 (RFC 1869) 8BITMIME extension. Despite this, some MTAs, 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 RFC 6152. This "just-send-8" attitude does not in fact cause problems in practice, since virtually all modern email servers are 8-bit clean.
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