Sending email to club members as mass mail 30% Undelivered Mail Returned to Sender

This has just started to happen in the last 2 days.

Email sent as mass mail to approx 80 people Undelivered massages for about 22 people, in 3 common Australian domains.

Error return message details similar to this

This is the mail system at host

I’m sorry to have to inform you that your message could not

be delivered to one or more recipients. It’s attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can

delete your own text from the attached returned message.

The mail system

address: host[]

said: 550 Rejected - Content Policy (in reply to end of DATA command)

550 Rejected can mean a spam filter on the server has rejected the message, but you will need to contact your email provider, who will be able to tell you more about why they rejected it.

Thanks Gary,
I was sure that this was the reason, but I don’t understand why this has just happened on 2 different domain email servers. I have also emailed the previous rejected users from my personal Gmail account using mass mail and the emails have go thru no issues so I guess that it has something to do with my web/email service provider that I am using. I have raised the issue with them. Thanks for your reply.