And if you want to improve even further upon that code snippet, you go for a fast numeric loop instead of a slower property enumeration loop.
js code:
var Contacts = ["johnsmith@hotmail.com", "abcdefg@hotmail.com"];
 
function OnEvent_Signin(Email)
{
>>>    for (var i=0, len=Contacts.length; i < len; ++i)<<<
    {
        if (Contacts[i] === Email)
        {
            // <snipped>
        }
    }
}
Or with a reverse while loop:
js code:
function OnEvent_Signin(Email)
{
>>>    var i = Contacts.length;<<<
>>>    while(i--)<<<
    {
        if (Contacts[i] === Email)
        {
            // <snipped>
        }
    }
}