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When sendmail is given the -d debugging switch, it internally performs three distinct actions. First, if the category.level is omitted, sendmail presets all categories, 0–99 inclusively, to a level of 1. It then sets the categories in the command line (if any) to the corresponding levels specified (or to 1 if no level is specified). Finally, it calls setbuf(3) to place the standard output in unbuffered mode.
Setting categories 0–99 to a level of 1 has two side effects: