Update - Yahoo have responded to a third follow up request with exactly the same canned response. We are still seeing some email being delayed delivering to Yahoo email servers and a small number of failures.
Jan 10, 09:56 GMT

Monitoring - Changing the status of this issue to 'Monitoring'
Jan 6, 11:21 GMT

Update - We are continuing to monitor the situation. While a lot of bulk emails are being delivered successfully to Yahoo, we are still seeing some continue to be delayed and fail. We are still awaiting a response from Yahoo support.
Jan 6, 11:21 GMT

Identified - Our engineers have identified an issue with all bulk email being prevented from delivering to Yahoo email servers. This affects email addresses with the following domains:
- yahoo.com
- yahoo.co.uk
- aol.com
- rocketmail.com
- sky.com

Only our bulk email route is affected (emails sent from workflows and bulk actions). Transactional emails continue to have very good deliverability.

We are in contact with Yahoo email support to try and resolve this issue and we have taken action to attempt to route around the problem. Bulk email is now being delivered to Yahoo, though some emails are significantly delayed and a small number are still failing.

We will keep you updated with the situation.
Jan 5, 08:01 GMT