Never send out wedding invitations over the internet using either an attachment or an e-card, especially if you are having a formal type of wedding. Traditional hand printed envelopes through the mail are still considered to be the proper way to send a wedding invitation.
Make sure to double check all addresses before mailing out your invitations. If you aren't sure of an address call the individual or send an email for a confirmed address, this will save time and expense in re-mailing a returned invitation.