sendmail, 4th Edition

Continuation Lines

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.