Answer: These days, believe it or not, email (or e-mail) is a Spanish word, very commonly used, although it has not been recognized yet by the Spanish Royal Academy and is considered by many to be an anglicism. It even has a verb form, emailear, that is sometimes used. It is one of those English words that has been adopted into Spanish even though some perfectly good "real" Spanish alternatives exist.
In fact, some of those alternatives also are used. I have seen both the term correo electrónico (which is listed in the RAE's dictionary), the term I prefer, and its shorter cousin, correo-e used, although email is also quite common (and is often pronounced pretty much as it is in English, although the final l sound is more like the "l" in "light" than like the "l" in "mail"). If you're talking with someone familiar with the Internet or computer technology, any of the terms are likely to be understood.
The example of email isn't an unusual one. Many Internet and other technology-related terms as well as words from popular culture have been borrowed from English and are used along with "pure" Spanish counterparts. You'll hear both browser and navegador used, for example, as well as both tráiler and avance for a movie trailer or preview, with the former probably being more common (although the written accent isn't always used).
Incidentally, while many of the sentences used as examples on this site use straightforward "textbook" Spanish, many others are taken directly from contemporary Spanish publications or Web sites. Because of that, some of the sample sentences may not always be acceptable to purists, but they do represent the way Spanish is written or spoken in real life.
As to plurals, it is very common in Spanish for words that are imported from foreign languages, usually English, to follow the same rules of pluralization as they do in the original language. For many words taken from English, then, the plurals are formed simply by adding an -s even if an -es would normally be called for according to the rules of Spanish orthography. One common example, at least in Spain, is that the Spanish currency, el euro, is divided into 100 cents, not the centes you might expect.