Yahoo recently updated their DNS to have a DMARC record that let’s ISP’s know to reject any mail with a Yahoo email address as the “from address” if it is not coming from Yahoos mail servers.
That means that you can’t use your Yahoo email address as the from email with our service (or any service that isn’t Yahoo)  If you do, ISP’s that respect DMARC records (Gmail, Hotmail, etc..) will bounce your email.
We have updated the campaign creation process to alert users should they try to use a Yahoo email address.  We would also suggest to avoid using free email addresses as your “from” email in general.
If Yahoo changes this in the future we will update things immediately to allow you to use a Yahoo email address as a “from” email again.
Learn more about DMARC and the Yahoo changes