Ah, the ever-so-convenient-but-sometimes-offensive mode of communication. Below are a few times when you should use email, and a few times when you shouldn't. (This also applies to other communication mediums like Facebook messaging.)

Yes

As a means for the guests to RSVP. Provide your email in your invitation as an option, especially if you can't afford the postage for a snail mail RSVP.

No

As the invitation. Unless you're using an online service like Evite, don't send emails as the invitation to your wedding. As a proper and important occasion, it should be treated as such. If you wouldn't throw out the invitation as part of a random verbal conversation, you shouldn't do it in an email.

Yes

As a way to send information to your wedding party and other important people.

Things like dress rehearsal times and what the bridesmaids should do for their hair.

However, always make sure everyone in the address list is on the same page before you send out a mass email.

No

As a private communication thread that started as a mass email. Unless the bridesmaid's reply to your email about dress fittings is something everyone needs to know, Reply All's are annoying and unnecessarily fill up the inbox.

Yes

As a way to get mailing addresses for the invitations. In this day and age, people just don't have Rolodexes any more.

No

As a way to deal with sensitive issues. Even if it's a private message, you shouldn't handle miscommunications and hurt feelings over email or messaging, because it creates more miscommunications and hurt feelings. Do it in person, or at least over the phone.