Teaching at Salzburg University of Applied Sciences there is a heavy focus on business English and, of course, e-mail writing.
Across the internet, there is a range of spellings for e-mail, but I will argue, till I’m blue in the face, that it must be spelled with a hyphen: e-mail.
When e-mail started out it was written with a capital, now you just look old-fashioned if you use a capital for it (and a bit daft, after all, you don’t use a capital ‘L’ for ‘letter’, do you?!).
Reuters says that e-mail should be ’email’, without a hyphen, but what happens when we talk of ebanking, ecommerce, eentertainment, egreeting? According to Reuters, these should all have a hyphen (becoming e-banking, e-commerce and, thankfully, e-entertainment, e-greeting). So why should e-mail be different? If we have a rule, surely it’s easier to apply it to all the words that are likely to need it, than to select one that should break the rule.
OED, in its benevolence, says that you can use either, but email, without the hyphen, is the default.
I think it’s a shame. Here we are at a massive moment in the English language; it’s changing and evolving, and it’s a great opportunity to rid the language of all these silly exceptions to rules that confuse both native-speakers and learners of English.
While the definitive answer is still in flux, I will continue to teach that it should be spelled with a hyphen, so that there is some hope of maintaining consistency within the rule.