-U

-U

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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.