Huge problem with Shaw and Apple Mail. Shaw now insisting I must use Shaw webmail.

Shaw is my internet provider. I have always used the Mail app on my computer. All of a sudden Shaw says they do not support apps other than Shaw Webmail for emails. I have spent at least 24 hours on Shaw's help line in the past few months. They keep referring me to AppleCare which has been somewhat of a help but it is Shaw's problem. Has anyone else run up against this situation? The odd thing is that this only affects my email and ONE friend's! I cannot send myself an email without it being marked (SPAM). If I email her from Mail she does not receive it so we have had to use Shaw Webmail which is inferior in every way.