I have recently upgraded to a new imac and setup my email as usual. Longtime user of both mac and Shaw and have always had "rules" applied to incoming email to send them to specific folders depending on who they're from.
Now any email which gets moved to a folder after it's received on my mac is automatically deleted from the Shaw server.
In doing so, the email doesn't sit around long enough to get sent to my iphone. And it's gone from the server if I ever had to remotely log into webmail.
Other mail is left untouched on the server.