You must use “Mail Manager” (linked from the left side of this page) and re-create the settings for mail boxes and forwarding addresses that you have with your existing provider.
You should set up your mail boxes in your e-mail program (Outlook, Thunderbird, Eudora, etc.), and check that they are working. Your e-mail program will allow you to check both your new and old accounts (i.e. those with your previous provider and those with Simba), so that once your settings are ready, you will not have to worry about losing mail as it will come to one or the other.
Mail will only start being delivered to your mail boxes at Simba once the relevant name server record is transferred (see the relevant question for more information). It is important to have your settings ready so that you do not lose mail, which could happen if the Internet starts sending your mail to Simba but Simba has no settings for you.