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:
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):
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.
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:
So in general the usage for which you are using it is correct from a grammatical standpoint.
That said, common usage is that when "RE:" is the beginning of an email subject line it specifically relates to a reply, and using it otherwise will probably be confusing to email users, many of whom may not be familiar with the term's history or denotation.
It would likely be less confusing if you were to use it in a subject line of an email, but not at the beginning:
"My comments re: suitability of
unicorns in marketing materials"