When your credit reaches zero, we turn your account into a restricted user account. This limits your number of e-mails per day and reduces the amount of storage available to you. As soon as you add credit to your balance, your restricted account reverts back to a fully-featured account.
If you do not make any payment within 30 days, your user agreement with mailbox.org expires and we delete your account. All of your data held on our servers is irreversibly deleted after another 30 days.
You can add credit to your balance in many different ways: bank transfer, PayPal, Bitcoin, etc. In the mailbox.org Office interface, simply go to Settings (cog wheel icon) -> mailbox.org -> Add Credit to Your Balance.