Under old versions of
sendmail, all lines of text output by
sendmail (including the header and body of a
message) automatically have the high bit cleared (zeroed) for every
character. This behavior remains unchanged under V8
sendmail for configuration file versions 2 or
less (Section 17.5). But with version 3 and above
configuration files, the message body is transmitted with the high
bit intact by default. For those delivery agents that should not
allow 8-bit data to be transmitted, you can use the
F=7 delivery agent flag to force the old behavior.
Beginning with V8.7, the F=7 delivery agent flag
can be used to suppress certain kinds of MIME conversions. For
example, if the EightBitMode option (EightBitMode) is set to p (pass 8) and
if the message contains 8-bit MIME data in its body, this
F=7 delivery agent flag will force
sendmail to bounce the message with the
following SMTP error, and to exit with EX_DATAERR:
554 5.6.3 Cannot send 8-bit data to 7-bit destination
Note that F=7 affects only the message body.
Headers always have the high bit cleared.