You can always add more email addresses to your account by selecting additional plans. The price will be pro-rated to sync with the expiration date of you original Email purchase. For example, if you purchase additional email accounts one month after your original purchase, the price will be pro-rated for 11 months of service.
If you are a reseller using our API, you can purchase any number of email addresses. However, your prices will not be pro-rated at this time. Instead a new bundle will be created for your purchase.