When you break up with a girlfriend, contact doesn't always end at once. In today's world, there are many ties to someone you once dated - both electronic and otherwise. Emailing, text-messaging, or even IM-ing an ex girlfriend might still occur even after you've ended your relationship. Sometimes this communication can be platonic or friendly, but other times it can be geared toward getting your ex back.
First, you need to know why you're staying in touch with your exgirlfriend. Being friends with your ex is one thing, but having an agenda in mind is quite another. You need to be honest with your own intentions. If she dumped you, or the relationship ended before you wanted it to, there's a good chance you might want to fix your breakup. If this is the case, there are ways you can get back together with your ex, as long as you begin working toward that goal.
Immediately after breaking up is not the time to start emailing your ex. It's a time for reflection, for the both of you. Left alone, your girlfriend will get a solid dose of what her life will be like without you in it. She'll miss you this way, because by not calling or emailing her she's cut off completely from that type of contact. Staying in touch with her however, will prevent this from happening. Your girl can't miss you if you're still around, which is why leaving the computer off for a while is probably your best bet.
In trying to get back your girlfriend, email isn't the only way of communicating. Many people also use Facebook or MySpace to keep in touch. If you've been writing on your ex girlfriend's home page, you need to stop this type of communication also. At least for a while. Giving her some breathing room is the best recipe for reconciliation. Remember... neither one of you are going anywhere. Your goal is to make your ex miss you and want you back... and this can't happen while you're always leaving her messages, notes, or emailing your ex girlfriend.
After enough time has passed, email is a pretty simple way of reestablishing communication. Contact ex girlfriend etiquette requires that you have a legitimate reason for getting in touch with her, and e-mail can really help with that. You could be passing along a funny joke or forwarding a cool video that one of your friends already sent you... and along with it, a little note asking how she's doing or what she's been up to. This type of innocent communication is a great way to get your ex to call or contact you, especially if you've been apart for a while with no contact. Making your ex girlfriend curious about what you yourself have been doing is another way of getting her to email you, but again, it requires giving her some space first.