The short answer is that payments of small amounts will have a relatively large percentage of fees deducted as a processing fee. While a typical email account may cost very little over the course of a month, the processing fees would be a large percentage of the cost. Accounting for processing fees, the 6-month payment option ends up not being much more than what a monthly option would be.

A Mailbox is the entity which receives mail. It is tied to an email address, but can recieve mail for multiple addresses/aliases. You could think of this as a physical mailbox where both you and your spouse's mail gets delivered to the same location.

An alias is another email address whos mail gets delivered into the same inbox. Having aliases can be useful to providing a 'throwaway' email address to a company if you are not sure they will securely handle your personal information. If an alias address is leaked and receives spam, it is easier to get rid of the alias that you have provided to one company instead of getting rid of an email address that you have provided to friends and family.

Alias creation. Restrict users of the subscription from creating aliases for their mailboxes, an limit this feature to Admins.

Create "approved contacts" list, and restrict a mailbox to only interacting with this list. (Parental Control)

Admins can view a graph of the account's send/receive traffic, and can break this down by individual mailbox.

Admins can also view statistics of the Storage used per mailbox

Normal users can view a graph of their mailbox's send/receive traffic over time.

We will keep your mail on our servers for the specified duration depending on which Subscription is purchased. After the duration has passed we will remove the email from our servers regardless of whether it has been read or not. If you only use our webmail interface, this means you will not be able to see emails which have been removed. We suggest that you use a email client to download your emails.

Each account/mailbox has a limit to how many emails they can send each day. The limit runs over the course of a day from 00:00:00 UTC to 23:59:59 UTC.

Example 1: I have a Personal subscription - I have 1 mailbox so I can send 50 emails per day from this mailbox.

Example 2: I have a Family subscription with all 5 mailboxes activated - My account can send 250 emails per day, so each mailbox is limited to 50 emails per day.

Example 3: I have a Family subscription with only 2 mailboxes activated - My account can send 250 emails per day, so each mailbox can send at least 50 emails per day with 150 "flex" emails left over. The 150 flex emails can be sent from either active mailbox in the account.

Example 4: I have a Business subscription with 20 mailboxes activated, and 1 AdditionalSend addon purchased - My account can send 20,000 emails per day (10,000 from the Business subscription, and an extra 10,000 from the AdditionalSend addon), where each mailbox can send at least 100 emails per day. With 20 activated mailboxes, this leaves 18,000 flex emails that can be sent per day from any of the active mailboxes in my account.