hi!
In my mind, "an email" is correct. Have a look in a Dictionary and you can find after the word the pronunciation.
email = ['i:maIl]
You can see email is pronunced as "i:maIl". Thats the reason why you should write "an email".
Have a look at your word. Is the first letter one of these (a,e,i,o,u)
and the pronunciation is the same than you can write "an" but is the
first letter of pronounciation not (a,e,i,o,u) than you must write "a".
a little exercise: -> a or an <-
1) ... author
2) ... British author
3) ... great authority
4) ... hotel
5) ... Amercian hotel
6) ... European hotel
7) ... hour or two
8) ... quarter of ... hour
9) it's ... honour for me
10) it's ... great honour for me
11) ... university
12) ... united Europe
13) ... unusual university
14) ... usually practised method
15) ... unborn baby
16) ... excellent author
17) ... European country
18) ... honourable man
=)