The Mailbox Admin allows you to edit your User Options, including aliases. To manage your Mailbox you must login with an email address that owns a Mailbox.
Attempting to login with an alias will generate an error because the alias does not actually "own" a Mailbox. This is because the alias is not a physical Mailbox, it is only a virtual pointer to a physical Mailbox.
For example:
The following email addresses actually share the same physical Mailbox.
user@domain.com is a physical Mailbox and is also the Mailbox owner; this is the login needed to manage the Mailbox.
user@domain.org only points to user@domain.com
DOMAIN.COM is the primary domain name.
DOMAIN.ORG is an alias or a virtual pointer that points to DOMAIN.COM