So says the Chicago Manual of Style which is the reference used by almost all newspapers and magazines, especially, but not only, in the Unites States of America. Note the following from section 7 [7.90] page 8 of Edition 15 of the Chicago Manual of Style.
So says Microsoft’s Manual of Style for Technical Publications under “E” on page 252.
It’s very simple really. E-mail is an accepted abbreviation of electronic-mail.
Having made that perfectly clear, if you are someone who does technical documentation and you decide that both the Chicago Manual of Style and Microsoft’s Manual of Style for Technical Publications have got it wrong, and you want to use ‘email’ (when you really should be using ‘e-mail’), then at least use it consistently throughout the work.
I decided to post this clarification because I have recently had to check (edit) some technical documentation and the author uses both email and e-mail in the content. In two places email and e-mail are used within the same paragraph.