A line that begins with either a tab
or a space character is considered a continuation of the preceding
line. Internally, such continuation lines are joined to the preceding
line, and the newline character of that preceding line is retained.
Thus, for example:
lions and bears
line begins with a tab character
is internally joined by sendmail to form:
DZzoos\n lions and bears
newline and tab retained
Both the newline (\n) and the tab are retained.
When such a joined line is later used (as in a header), the joined
line is split at the newline and prints as two separate lines again.