Are you sure you mean the ampersand - & - sign. This is simply short for "and", and as far as I am aware, isn't used much in email addresses. You're probably thinking of the @ sign, which pre-email was used in shops, markets and in accounting, where it meant "at" - eg "2 shirts @ �10 each"; of course, it still means "at" in email addresses.
Just in case there's any confusion. @ is NOT the ampersand sign, which is &, and stands for "and". There is no fancy name for @, which stands for "at"; most people in the UK seem to simply call it the "at sign".
I am confused. What has the ampersand got to do with email addresses? It is derived frokm "and - per se - and" (i.e. and in itself) and it originated as a sort of squiggly combination of "e" and "t" ("et" being the Latin for "and").