Make sure you're using your correct username and password. Remember the password is case-sensisitive. If you have trouble remembering your password or username, contact support and we can send you your access information (only to the alternate email address set in your Control Panel settings - so make sure that's a current working email address.
Make sure you're trying to log in to the server on which you have an account. If you signed up for a USA-hosted account, you won't be able to log in on the Netherlands servers. The USA servers are at neomailbox.com and the Netherlands ones at neomailbox.net or neomailbox.nl.
Just send us a mail if you can receive mail but cannot send mail or access the Control Panel, and we'll re-activate your account and increase your limits if necessary.
If you expect to be sending mail in larger quantities than the default limits for new accounts allow, it's a good idea to let us know beforehand and have your limits raised. That will prevent your account from being disabled if you go over the default limit.