This is one of the simplest things I am going to cover as it consists of a single file.
In the missions directory there is a file called email.txt which contains the information on when the e-mails "arrive" as well as the actual e-mails.
Each e-mail consists of the following elements.
First line is the mission number the e-mail is displayed after.
Second line contains who the e-mail is from.
Third line is the e-mail title.
Fourth (and possibly fifth, sixth, seventh etc.) line is the body of the message. I have noticed that there can be a fair amount of text per line of this, and up to six lines (for paragraphs) have been used. This should be plenty of text.
Whether you use e-mail in your campaign depends on what you are trying to do. Since it does not add a huge amount to a campaign you may feel that it is not worth doing if you would have to redo the entire cabin for it or risk spoiling the atmosphere of your campaign by having the standard look hit the player in the face.