The hyphen in e-mail.

[Jorge Junior]

Hi. I'm Jorge, from Brazil.

The spelling "email" is not correct, is it?
If it's correct, so no problem if I write xray, hbomb, ecommerce... Am I right?

In this case, the hyphen is used because it's a compound noun where the first element is an adjective, right? Or there's another reason?!
What does grammar say? Please, help me.

[Mister Micawber]

.
The use or disuse of a hyphen in email/e-mail/ E-mail does not affect its use in other words. Each evolves separately; the usual order of evolution is from two words to single hyphenated word to single unhyphenated word, and this evolution happens with increased use.

Email can be found in several acceptable forms. Here is the comment of the MLA:

There are numerous spelling variants of this word. In Internet traffic up to 1995, `email' predominates, `e-mail' runs a not-too-distant second, and `E-mail' and `Email' are a distant third and fourth. (Modern Language Association (MLA):
"email." Jargon File 4.2.0. 02 May. 2008. <Dictionary.com>.
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_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber