Have you experienced the frustration of sending a message to a friend only to find it bounced back to you? Your friend has probably changed their email address and forgotten to tell you about it. If it's someone you see at the pub or at work, or can contact by phone, you can just ask them to give you their new address. If email is the only contact you have, however, it becomes a lot more difficult to stay in touch.

The internet has brought us tremendous freedom of communication and competition between internet providers and email providers means that it's never been easier to run several email addresses or to change to a new address when you feel like it. Usually, you'll send out an email to let people know that you have a new email address, but it's not unusual for people to be missed off the list and lose your details.

So once you discover you can't contact your friend by email, what do you do? You can try mutual friends to see if they can pass the address on, or you can try reunion sites that help you to get in touch with old friends from school, college, work, the forces or any other groups that you might have belonged to. For that to work though, your friend has to be registered and there will usually be a subscription charge to pay before you can contact them.

Similarly, if you've recently changed your own email address, how do you make sure you've told everyone, or that you can catch people who try to get in touch after a few years of no contact?

The ideal solution is also on the internet: services that let you set up a single account for all your old email addresses. People can find you via your old details and you can be sent a message letting you know that someone's trying to get in touch with you. Whether you respond is up to you – after all, there may be some people that you don't want to renew your friendship with. But for most people, online services like these help you to ensure that everyone who wants to get in touch with you can; or that you can find someone you thought you'd lost touch with for ever.