E-post can mean both the message itself and also the address you send it to, like in English? Jeg kan sende en e-post til e-posten din?
Yes, that is quite common! Or you could say "e-postadresse", if you wanted to be very specific.
Ugyldig - i put unacceptable but it said it was wrong. Is that right? Have i misunderstood gyldig/ugyldig?
It specifically means invalid, whether legally or in the sense of data.