To set up any additional email address (not just Posteo) in Windows 8 Mail, you require a Microsoft account. This needs to be set up before you can use Mail. If you do not want to do this, you can use Thunderbird instead of Windows 8 Mail. The instructions for Thunderbird are found here.

Written instructions

1. To set up a new account, start Mail (the program) in Windows 8 (by clicking the Mail tile), move the cursor to the right edge of the screen and open “Settings” (by clicking the cog symbol).
2. Go to “Accounts” and then “Other account”.
3. Select “IMAP”.
4. Enter your Posteo email address and password.
5. For “Username”, enter your Posteo email address again, and for the incoming and outgoing email servers enter “posteo.de”.
6. By clicking “Connect”, the setup is complete.

Setting up alias email addresses

Windows 8 Mail does not support alias adresses. To use aliases, please revert back to Thunderbird.