Some people are always rude, but more interesting are those people who are rude in email communication or while driving, for example, who would not be rude in person.
I received a rude email today from someone with whom I had previously worked well. When I tried to call the person to clarify and discuss the issues he raised, he refused to talk to me. I do not believe he would have been so rude in person.
Anyone with a driver's license has experienced road rage coming from someone who would not be so brave or aggressive in person. That person is content to gesture wildly and yell profanities because there is a large steel door between them and the recipient of their menacing tone.
The next time I am tempted to vent in an email, I hope I pause and remember how I felt receiving the email I did. Hopefully I will choose courtesy over unnecessary rudeness.