To create or edit your "From:" name and signature:
Sign in to your account at http://mail.yourdomain.com/ (be sure to replace "yourdomain.com" with your domain name). You'll be prompted to enter your AT&T Yahoo ID and password if you haven't already.
Click "Options" in the upper-right corner of the page, then choose "Mail Options" from the pull-down menu.
On the page that opens, click "Accounts."
Click the "Add or edit an account" link on the Accounts options page.
The Mail Accounts page open in a new tab or window.
Click the account that you'd like to update, then click the "Edit" button.
In the "Name" box, enter your name exactly as you would like it to appear in the "From:" line of your outgoing messages. Your "From:" name may not contain special characters, such as the @ symbol.
Note: You must enter a "From:" name. If you leave this box blank, the previous entry or your default name (the name you gave when you registered with Yahoo) will appear, and your other changes will not be saved.
Next enter your signature in the "Signature" box. You can include your name, contact information, or whatever you'd like!
Tip: This option only applies when the email account is your default email account.
Click the "Save" button to activate your new settings.
When you're done editing your account, we recommend closing this window (your new Business Email interface is still open in another window or tab).