CiviBuild - wp-demo install error with postdrop
Just found the fix that my postfix was not running.

I had to do a sudo postfix start which resulted in a number of warnings with final output 

postfix/postfix-script: starting the Postfix mail system

After this I retried running the civibuild install

civibuild install "civiwpbuild" --url "http://localhost:4000" --admin-pass "XXXXXXXXXX"

and it worked.

More details on postdrop: http://www.postfix.org/postdrop.1.html