They probably will also give you the opportunity to add 3 or 5 address you choose to the mailbox.
In most cases your ISP will give you one mailbox with one address they choose for you (often this is an abbreviation of the name you are registered with for their services or your username). Just like a single person can be reached on more than one phone number, a single mailbox can be reached via different email addresses.
This guide describes the definitions and affects of aliases on Outlook. In the corporate world, it is also not uncommon to have addresses from ex-employees or special projects assigned to your (Exchange) mailbox. However, nowadays, several e-mail providers allow you to set multiple addresses on just one mailbox. Normally a mailbox only has one e-mail address.
