error 6.6.0 sending mail

Help please, I have tried several times to send bulk messages, but each time they don’t go and I get the message as follows,  error 6.6.0 can’t send, it says due to something wrong with the body of the message.
I need to send messages to all my club members.

Barbara, did you resolve this? I am also getting it

Seems to me as if yahoo refuses the email.

Is there something special in the body of your mail? An image with invalid format, an attachment, something with big file size?