As of early April 2014, Yahoo has began imposing a stricter email validation policy to block email spoofing attacks on yahoo.com email addresses. This change means that platforms and apps that can send emails on behalf of your Yahoo email address would be rejected, because they would be deemed to be 'fraudulant' since the origin is from non-Yahoo servers.
Read more about this change here.
Yahoo mail offers the following domains as email addresses:
Currently, only email addresses with the yahoo.com (and possibly regional yahoo) domains are affected by this policy. As the yahoo.com domain is also representative of Yahoo's executives and staffs email addresses, this enforcement was done to prevent spammers from masquerading as Yahoo employees.
Read more details about this here.
Changing your TackThis email to a non-yahoo.com email address would resolve this issue. This is also the solution suggested by other affected platforms such as MailChimp.
Drop us an email via your current Yahoo email, with your new non-Yahoo email address to change your TackThis email.