If you deleted an alias address in your account, then you can add it anytime within protection period after you deleted the alias. Only after this period has passed this alias becomes available for the registration of other users.
The protection period is at least 90 days, depending on the age of the account.
You can view and edit the list of deleted aliases as per the below screenshot:
If you terminate your mailbox-account, your main address is blocked for at least 90 days, depending on the age of the account. Only after this period users can register this address again.
Suspension of your account due to insufficient credit
Please refer to this article: https://kb.mailbox.org/display/MBOKBEN/What+happens+when+my+credit+runs+out