The following resources might be interesting if you'd like to go deeper into the subjects of this chapter:
- Profile and Bashrc: https://bencane.com/2013/09/16/understanding-a-little-more-about-etcprofile-and-etcbashrc/
- Set up a Mail Transfer Agent with postfix: https://www.hiroom2.com/2018/05/06/ubuntu-1804-postfix-en/