Sask Tel Emails

As we are doing invoicing we have noticed some with SaskTel emails they seem to be rejecting, not all of them it seems to be pretty random the reject description says description says:
*The response from the remote server was:
This message has been blocked for containing SPAM-like characteristics. *
If we normally email you and you have not gotten your invoice just let us know.
Thank you and sorry for any inconvenience.