Electronic mail has several English spelling options:
email is the most common form used online, and is required by IETF Requests for Comments and working groups and increasingly by style guides. This spelling also appears in most dictionaries.
e-mail has long been the form that appears most frequently in edited, published American English and British English writing as reflected in the Corpus of Contemporary American English data and style guides.
mail was the form used in the original RFC. The service is referred to as mail, and a single piece of electronic mail is called a message.
EMail is a traditional form that has been used in RFCs for the "Author's Address"and is expressly required "for historical reasons".
E-mail is sometimes used, capitalizing the initial E as in similar abbreviations like E-piano, E-guitar, A-bomb, and H-bomb.