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:
DZzoos 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.