Log in to your 123-mail account. Enter your username and password into the boxes provided, then click the Login button.
From the toolbar at the top of the page, click the Configuration button.
From the left hand toolbar click the Accounts link from the Email sub menu.
Click the Add button at the top of the page.
Enter the relevant information into the boxes provided. Then click the Save button.
Your Yahoo account will now be added to your 123-mail account.