SMTP module: The following From address failed
I believe the problem lies with Yahoo not having your permission to send from the gmail account.  You could try setting up Yahoo to send from your gmail account but I'm not sure Yahoo will do that.

You could look into a mail service like MailJet, which is cheap and scalable.

Or, assuming you're using the drupal_mail function, simply set your $from variable in 'params' to 'my_yahoo_mail_address@ymail.com'.