word choice - "Email" or "e-mail"?

Which way of writing the word: "Email" or "e-mail" is correct? Both variants seem to be in wide use. If both ones are okay, maybe there is a difference in contexts they have been used (one is more formal than the other)?

Answer

Both e-mail and email are in standard use at this point, although e-mail retains a vast majority of usage in edited, published writing according to my research using the Corpus of Contemporary American English (COCA).

Here are the current results counts in COCA for various categories of English:

           e-mail  email
spoken     3535    711
fiction    789     285
magazine   5421    471
newspaper  6046    192
academic   3675    897
total      21696   2831
total(excl spoken)
           15931   1845

Obviously the “spoken” totals don’t represent any kind of actual usage but rather the policies of the organizations that transcribed the spoken data, so I also included a total that excludes the spoken examples. So, in (edited, published) fiction, magazines, newspapers, and academic writing we find that the traditional e-mail outnumbers incidences of email by more than 8.5 to 1. COCA includes data starting from 1990.

Now that we have established that e-mail retains the position of preferred usage by a very large margin, let’s look at trends over time. These numbers are in incidences per million words (rather than total incidences):

     e-mail  email
1990  0.15    0.00
1991  2.46    0.05
1992  1.67    0.00
1993  5.49    0.05
1994 10.40    0.24
1995 16.11    1.71
1996 22.41    2.14
1997 51.71    2.09
1998 38.15    1.62
1999 50.33    4.67
2000 85.98    6.37
2001 83.75    7.03
2002 95.29    3.38
2003 94.35    5.23
2004 79.68    4.51
2005 72.53    5.58
2006 72.38   10.09
2007 85.20   14.18
2008 68.26   26.56
2009 97.37   33.23
2010 87.93   26.66

This is a graphical representation showing the data over time:

The blue and red lines show the frequency of incidence for e-mail and email per million words. The orange line shows the ratio of incidence between e-mail and email over the same period. From these results, we see that e-mail was on a meteoric rise in the 1990s and by 2000 it has locked in at between 70 and 100 incidences per million words. Email, on the other hand, saw very little usage until 2005, when its use soared up to about 30 incidences per million words over the last few years.

What does all this tell us? We see a word whose spelling is in transition. It is not clear whether email will eventually reach and surpass e-mail. For the time being, e-mail has retained its position as the preferred usage by a factor of three to one over the past few years. The numbers can change very quickly and email may win out in five years, or it may stay a minority usage for a long time. Only time will tell. In the meantime, you are in good company if you use e-mail. That’s what I use.

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Edit March 8, 2012:

I created a Google N-gram which compares instances of e-mail with email in the Google Books corpus between 1988 and 2008, and it does seem that e-mail hit a peak around 2004, and has been in decline since, though it remains significantly more common than email.

Curiously, when you look in the country-specific corpora, you find that the gap is wider in American English and narrower in British English:

American English

British English