问题
what's 8bitmime? waht's the defference of 7bit and 8bit?
How to understand them?
回答1:
SMTP was originally specified using the pure ASCII character set. What a lot of people forget (or never get taught) is that the original ASCII is a 7-bit character set.
With much of the computing world using octets (8-bit bytes) or multiples thereof, some applications started, very unwisely, using the 8th bit for internal use, and so SMTP never got the chance to easily move to an 8-bit character set.
8BITMIME, which you can read about in excruciating detail in RFC 1652, or in a decent summary at wikipedia, is a way for SMTP servers that support it to transmit email using 8-bit character sets in a standards-compliant way that won't break old servers.
In practice, most of the concerns that led to this sort of thing are obsolete, and a lot of SMTP servers will even happily send/receive 8-bit character sets in "plain" SMTP mode (though that's not exactly the wisest decision in the world, either), but we're left with this legacy because, well, "if it ain't broke" (for very strict definitions of "broke")...
来源：https://stackoverflow.com/questions/3384065/whats-8bitmime