My firm (@sintons.co.uk) is unable to email accounts with a blueyonder.co.uk domain as they seem to be getting flagged as spam. If checked our domain on the MX Toolbox against spam filters and it isn't getting flagged. We have been able to send to the addresses previously, can you shed any light on the problem?
Response received -
This is a delivery failure notification message indicating that an email you addressed to email address :
Welcome to our forum, I am sorry we were a bit slow responding and you are having problems emailing our blueyonder domain. We are presently taking steps to deliver suspected spam email to our customers, allow them to decide on its legitimacy and then update our filters, more information here Spam Folder Settings Change
It may take us a few weeks to fully implement but once done fingers crossed it will resolve your problem, if not do let us know and we will be happy to investigate further.
I am sorry this is still happening for you. We have many mailboxes as I am sure you can imagine and the job is taking a little longer than expected. We are hopefully aiming to complete it early in January, we will do our best to keep the forum updated. Once complete if you are still getting NDRs we will happily investigate.