How to send an email from an external account while in Yahoo! Mail
This article describes how to send a message from an external email address via SMTP while in your Yahoo! Mail account.
To send an email from an external email account from within your Yahoo! Mail account, click the down-arrow at the end of the "From:" field when you’re composing a message.
If you already added an external POP email address to your Yahoo! Mail account, it should appear in the drop-down.