-U
This is the initial MUA-to-MTA submission V8.8 through V8.11
The -U command-line
switch is used to tell sendmail
that this is the very first step in this email
message’s submission.
From V8.8 through V8.11, this switch did nothing.
Beginning with V8.12, this switch was eliminated,
and the default behavior of
sendmail was changed. Now
sendmail presumes that any
message’s submission is an initial submission unless
the -G
command-line switch is present.
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