The -A switch doesn't cause sendmail to be built. Instead, it prints the architecture component of the path that will be used to build the program:

% ./Build -A
obj.SunOS.4.1.4.sun4

That is, each program will be built under this directory in a subdirectory named after the program. For sendmail, in this example, the path would be:

obj.SunOS.4.1.4.sun4/sendmail

See also the -M switch (-M).