What is an alias email account ?
When a user has multiple email address for the same account, the additional email address can be set up as an email alias to the account.
Email aliases are particularly handy for handling multiple domains. When an organization has multiple domains, say yourdomain.com and yourdomain.org, providing multiple email addresses becomes simple by adding aliases to the accounts.
There is also the case that you may want 2 main accounts and 4 different aliases that are connected to these 2 main accounts. During the activation process you will select 2 main accounts and not 6 (which is not also an option).