The full name is electronic mail and I have seen it written either e-mail or email.
Judith Kallos of Netmanners.com has written four books on the subject of e-mail etiquette so I asked her and she said the correct spelling is e-mail and she got her information from the "Chicago Manual of Style".
So there you have it folks. E-mail is correct!
Whichever way you choose to write it you need to be consistent throughout your document. Your company may even have a preference on how they want it written.