You probably don't need to migrate your email accounts very often, but a good reason comes up every once in a while. In those instances, YippieMove is a service that can help.
YippieMove automatically transfers all of your old emails -- attachments and all -- to a new account. This can be particularly handy, for example, if you have an old Yahoo! account that you would like to make a backup of in case your address is ever reset and given away. We've previously covered Backupify Migrator for getting data out of a Gmail account, but YippieMove allows you to transfer between any combination of a long list of email providers, so most people should be covered.