Do use email to than an interviewer if, knowing your tendency to procrastinate, your choice is email or nothing. Do use email when your instinct tells you it s the right conduit. Be friendly and personable but stick to business. Omit the joking or slang remarks that you might use with your friends, however. Don t use email to follow up with an interviewer who, based on interview appearance and behavior, you suspect is a traditionalist.
Paper works better with traditionalists.
They like the fact that you went to more trouble to print out your letter, buy a stamp and mail it. It may be best to compose your message using a standard word processor so you can check spelling and grammar. Then copy and paste the message to email. Poorly written correspondence, whether hard copy or electronic never leaves a favorable impression. Paper stands a better chance of being saved as a reminder of your savvy manners.