Permanent email errors sending to Yahoo and AOL domains

[#1]

since a few days ago, any email I send to a yahoo or aol email address returns an error: smtp; 554 Message not allowed - [PH01] Email not accepted for policy reasons.

It makes no difference if I send to 1 person or several people. The only email I have been able to send is an empty message with the subject Test.

Are Yahoo and AOL using some kind of blacklist that I have ended up on, and if so how do I ask them to change it?

[#2]

Thanks for getting in touch, I'm sorry for the delay in getting to your post and for the problems you're having with the mail. When trying to send the mail that is failing, are there any attachments at all?

[#3]

no attachments, just text - some emails only to 1 person are getting through, but group emails are always failing. I just tried using the Bcc field for all recipients and that seems to have worked.