An email address identifies an email box to which email messages are delivered. This article covers modern internet email, but many earlier email systems used different address formats.
The local part of an address (before the @ sign) is case-sensitive (with the exception of postmaster@example.com). The domain part (after the @ sign) is not case-sensitive. Most organizations treat uppercase and lowercase letters in the local part as equivalent. The risk of delivery failures due to case differences can be minimized by using only lower case characters when creating new addresses.