Unable to send message;Reason:554 5.7.7 Mail Policy violation : Check attachments and try again

Unable to send message;Reason:554 5.7.7 Mail Policy violation : Check attachments and try again

I'm getting this message " Unable to send message;Reason:554 5.7.7 Mail Policy violation : Check attachments and try again" when I try to send emails with "allowed" attachment file types.

I have defined a mail policy stating that eg.*.txt files are allowed. However when I try to send an email with a .txt extension i get the message " Unable to send message;Reason:554 5.7.7 Mail Policy violation : Check attachments and try again"

Can you advise what I'm doing wrong?

Thanks