Why Email Address Are Used In LOWER CASE

Computer programmers and web developers use lowercase for most of their programming or scripting because it is much easier to remember. Being humans, we may not always remember what we typed in capital or lowercase, and with case sensitivity, one error could cause the entire code to not work. Using both capital and lowercase letters makes the work harder. Using mostly lowercase, and keeping capital to a minimum, makes programming or scripting less difficult.

All email addresses are not using only lower case. The actual specification says that the local part (before @) of an email address may use any uppercase or lowercase English letter (a-z A-Z). It is case sensitive (john@email and John@email is not the same address), though this practice is discouraged and not common.