Why do we call snail mail "snail mail"?
So by default mail will refer to email?
"Snail mail" is an example of a retronym, coined to distinguish the old type of something (in this case "mail") from a newer meaning.
In this case, the retronym is disparaging: neutral alternatives could be "letter mail" or "post".
For some people it may be the case that "mail" will usually mean "e-mail", just as for some people "guitar" means "electric guitar" and "hockey" means "ice hockey". (I suspect this will be more true in the UK, where "post" is more common than "mail" for the traditional service). And in context, "mail" can certainly mean "e-mail".
But out of context it won't necessarily have that meaning.
No, E-mail is E-mail, which comes from Electronic Mail. "Mail" is the general term for "letters and packages conveyed by the postal system" (NOAD), although it can also indicate electronic mail depending on the context.
Snail mail, or Postal mail, is still the standard, ordinary mail system, but this expressions simply highlights its intrinsic slowness, being opposed to the E-mail that is fast.
There are 2 kinds of mail:
When somebody says “mail” they may be referring to one or the other or even both.
If most of your mail is email, you are probably referring to email when you say “mail.” If most of your mail is postal mail, you are probably referring to postal mail when you say “mail.”
To disambiguate the word “mail,” the term “snail mail” was coined to refer to postal mail. The relative slowness of postal mail delivery as compared to electronic mail delivery is analogized to the slowness of a snail. But it is not the case that postal mail has any more right to the word “mail” than email does. Same as landline phones do not have any more right to the word “phone” than cell phones do. I can write a letter to you without knowing how I will send it — then I can mail it to you. Whether I used electronic mail delivery or postal mail delivery is irrelevant. If we have a phone conversation today and I am on a cell phone and you are on a landline phone, we are still having a phone conversation.
You can see examples of email just being called “mail” in many places:
Also notice that prefixes like “electric” and “electronic” and “cyber” tend to get dropped once electricity, electronics, and the Internet (respectively) become ho-hum everyday things. So one can expect “electronic mail” to devolve into “mail” even if it was the only kind of mail.