It's rude. Did you send out wedding invites by e-mail or other digital means? This is not proper. Your grandmother and your mother would disapprove. If you're asking then you know it's questionable etiquette. Do not e-mail. Etiquette is not about convenience.
Even though the world has evolved electronically, it's frowned on to send 'emails' as thank you's especially for something as formal as a wedding. Weddings have always been about tradition and a formal celebration - making a handwritten thank you note a must.