If PHP mail() function doesn't works on your development desktop and your Apache error log shows something like "sh: /usr/sbin/sendmail: not found" - then you obviously need to install "sendmail" in order to fix that :)
Just run the following command in Ubuntu:
# apt-get install sendmail
To configure Sendmail for your needs use (see appropriate configuration scripts in output):
Writing this post for people who asking me quite frequently: "Why PHP mail() function doesn't works after I've successfully installed LAMP development environment on my Ubuntu desktop?"