Avoid unusual or eccentric fonts in business email. The font should generally be black, and neither too big nor too small. Arial is a common default font for email; it has a clean appearance and is easy to read. Other options include (but are not limited to) Verdana and Tahoma. Justifiable exceptions to these font guidelines might be found in a professionally designed signature file, created under the guidance of a corporate art department or designer. It is fine, though, to use the same font for your signature file that you use throughout the body of your email.
If you wish to depart from these recommendations for a particular type of email — a marketing message, for instance — seek the design advice of a qualified professional.