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In addition to this keep one thing in mind the account you've added will be scheduled on the top-left column. You can simply choose an account from that you want to compose or send an email from that account.
In case you are not able to send mail from an extra email address in yahoo then you can simply take help from a third party tech support providing company and get assured solutions in an efficient manner. You can also dial a toll-free yahoo helpline number to fix the issue.
http://www.contact-customersupport.com/yahoo-support/