message-mailer-swallows-blank-line

message-mailer-swallows-blank-line is a variable defined in `message.el'.
Its value is

nil

Documentation:
*Set this non-nil if the system's mailer runs the header and body together.
(This problem exists on Sunos 4 when sendmail is run in remote mode.)
The value should be an expression to test whether the problem will
actually occur.

You can customize this variable.