As soon as you've added more than one email account in Windows Live Mail, it will show a dropdown menu in the new message window, which allows you to select from which email address to send the current message. The "default account" is automatically selected, but others will be possibly be used when you are replying or forwarding messages (see first section, below). The most recent email address you've configured will optionally become the new default, but that's something you can change with just a few clicks. (Screenshot on the left shows how the default From address can be set when you add new email accounts).
Note: regardless of the current default sender, Windows Live Mail will always pick the email address to which an email was sent as "From Address" when you hit Reply or Reply to All. When you forward an email, the From Address it selects will also be the one to which that message was sent.
When you only have one account setup in Windows Live Mail, it is automatically the default - and selected when you compose a new message or reply to one. When you compose a new (blank) message, the default account will automatically be selected. Just click on another one in the listing to change the "From" email address: this only affects the current message, and future blank emails will still show the default sender automatically selected. Even when you reply to an email (recipient address is pre-selected), you can still change your From address: in other words, the reply can come from a completely different email sender!
Careful: when you compose a new email (either by pressing the Ctrl+N keyboard shortcut or clicking on the Email message button), Windows Live Mail will automatically choose the "From" email address based on what account / email folder is currently selected on the left. Even if Hotmail is not our default email account, the new message we created picked our Hotmail address as sender, because a Hotmail subfolder was currently selected:
To access your email accounts, click on the File button (top left corner), choose "Options", and click "Email accounts" on the right:
In the Accounts dialog Windows Live Mail just opened, you can tell which email address is currently the default (it says "
Default" between brackets). Click on the email account you now want as the default From address, and click on the "Set as Default" button: Click Close to return to your emails: you are done! The next message you send will automatically have the new default address pre-selected in the "From" dropdown menu.
Tip: you can also change the default email address with a right-click on its header (don't right-click on one of its email folders) inside the left pane, and by choosing "Set as default account" from the context menu: