How do I reply from my domain name email address?
You can reply to domain name email forwarded to your Yahoo! Mail account the same way you normally reply to any email sent to you. Please realize that when someone receives your reply, your Yahoo! Mail address (bob@yahoo.com) will appear in the "From" field and not your domain name email address (bob@domainname.com).
In order to reply to email with your domain name, you will need to upgrade your plan to Yahoo! Custom Mailbox or Yahoo! Business Mail.