The meaning is 'posta elettronica'; this is why it is considered feminine. Another meaning could be 'messaggio di posta', but the more general meaning is prevalent. This kind of rule does not always apply, for instance week-end is 'il week-end', and fine settimana is 'il fine settimana', or 'la fine della settimana', which -- by the way -- have different meanings.

It's right, but I don't know say u why e-mail is a female noun, perhpas for italian people the name sounds like a female noun. If it is male the sound not good (un e-mail o gli e-mail).