Your Choice of Domains
A customized email address will be built just for you or your company. If you always wanted your email address to be email@example.com, now you can have it.
It should not exist today. If you type it into the web bar on your browser, it should not find an existing page.
It should be unique to you and to your business. The more unique you want it to be, the more likely it would be available to you.
(Note: Setting up email for domains that already exist (and which you already own) can be done with the help of your IT staff. For reliability and avoidance of risk and confusion, it's easier if the domain does not already exist today.)