Under DOS and Windows, a message is normally stored on disk as firstline\015\012secondline\015\012...\015\012lastline.
Under UNIX, a message is normally stored on disk as firstline\012secondline\012...\012lastline.
In practice, it is very common for the last line to be empty. A partial line means a nonempty last line. Beware that some text-processing utilities ignore partial lines.
For example, the three-line message
This is a test.
... Three dots
is stored under UNIX as
"This is a test.\012... Three dots\012"
and under DOS as
"This is a test.\015\012... Three dots\015\012".