The data portion of an email message is transmitted during the DATA phase of an SMTP transaction. Prior to V8.6 the data were presumed to be 7-bit. That is, the high (8th) bit of every byte of the message could be cleared (reset or made zero) with no change in the meaning of that data. With the advent of ESMTP and MIME, it became possible for sendmail to receive data for which the preservation of the 8th bit is important.

There are two kinds of 8-bit data. Data that arrives with the high bit set and for which no notification was given is called “unlabeled” 8-bit data. Data for which notification was given (using BITMIME in the ESMTP session or with the -B8BITMIME command-line switch, -B, or with a MIME-Version: header in the message, MIME-Version:) is called “labeled.”

The EightBitMode option tells sendmail how to treat incoming unlabeled 8-bit data. The forms of this option are as follows:

O EightBitMode=key                  configuration file (V8.7 and later) 
-OEightBitMode=key                  command line (V8.7 and later) 
define(`confEIGHT_BIT_HANDLING',key)    mc configuration (V8.7 and later) 
O8key                               configuration file (V8.6, deprecated) 
-o8key command line (V8.6, deprecated) 

The key is mandatory and must be selected from one of those shown in Table 24-19. If the key is missing or if key is not one of those listed, sendmail will print the following error and ignore the option:

Unknown 8-bit mode char

Only the first character of the key is recognized, but we still recommend that ...

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