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.
Example: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org can all be the email addresses used by the same person under the same account. The email alias is similar to the base account email address, except that it can be removed and recreated by the admin easily.
NOTE: During the activation of Custom Email Service, you will be asked to insert how many main accounts do you want. As the aliases go under a main account, if you want for example one main account like email@example.com and two extra aliases connected to this account which are firstname.lastname@example.org and email@example.com then you have to select 1 main account.
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).