Netiquette
The informality of electronic mail poses a problem for the traditional norms of epistolary style.
In a formal e-mail message, there is nothing out of place in beginning with a formula such as Dear Professor Fillmore and closing with Very truly yours.
Since e-mail is a relatively new medium for communication, however, set phrases for informal greetings and closings are still being established. At times, the salutation and valediction are left out entirely, even when the correspondents do not know each other well.
Informal salutations include common greetings like Hi or simply the addressee's name. People have been much more creative with the closing, employing terms such as best wishes and cheers, the latter term previously associated with British use and perhaps adopted because it sounds a neutral note between the kind of closings used in letters and phone calls.
Still more informal is TTFN, an abbreviation for ta-ta for now, another Briticism.