Your User Email is Used to Login to Apollo and Your Linked Mailbox is Used to Send Messages
When you're invited to join Apollo, you'll receive an email with a link to create your login credentials. The email account that you receive the invite at will be used as your login email. If you want to update it after the fact, then you can do so under Settings > You > General > Account Info:
You can also update your password at any time in the 'Password' field, directly next to the 'Email' field.
Don't forget to click Save after updating!
Important Note: Changing your login email does not affect your linked mailbox.
Before you start sending emails from Apollo, you need to link a mailbox that you'll be using to send those emails. Keep in mind that each user is allowed up to 3 linked mailboxes.
When you're ready to link your mailbox, you can find it under Settings > You > Mailboxes -
For reference, more information on the setup process for your mailbox can be found here.