According to the Post onMail Support blog —
Coming Soon: @live.com and @windowslive.com domains
On June 20th, two new domains will be available for customers signing up for a new Windows Live ID: @live.com and @windowslive.com. These new domains will be available when you sign up for a new account with Windows Live Messenger, Windows Live Family Safety Settings, and www.live.com.
You don’t have to sign up for a new account…
but if you do, you will be able to use the account to sign in to any MSN or Windows Live service. The new account won’t contain your old mail and contacts, but we are working on a migration process that will allow you to transfer your current information to a new account.
Should I sign up for a new account?
If your current mail and contacts are more important to you than getting a new domain name, we recommend you stay with your account. Even if you do sign up for a new account you won’t lose your info, but we know it’s tought juggling two accounts.
If you are excited about the new domains names (we sure are!!) then definitely sign up and let your contacts know about your new address. This is a great opportunity to make get the account name you have always wanted (no more email@example.com).