My friend is having the same problems on his 24" iMac.
There IS a workaround, but it is NOT permanent, and he's having other du-plicate/triplicate email problems as well, all after Mac's last update (previous to the big 325 meg one).
With Mail open, go to preferences, then to accounts, then on the outgoing mail server selection box - open it and click on 'Edit SMTP Server List', then select the Shaw mail account, then the advanced box, if custom port is selected click on the one above, or if default port is selected click on custom port, then reclick the button that was previously selected, shut everything down, click save when it asks you, then your outgoing mail will work again, temporarily. It WILL screw up again till Mac stops trying to blame Shaw for the problem - it is a MAC problem, but it is disguising itself to look like it is originating from the server. It is NOT - the Shaw technician had me download Thunderbird email program, and we replicated exactly the same problem - for some reason, OS 10.6.6 is not remember the settings for the ports correctly, and this seems to be the culprit. Time to admit culpability and fix it, Apple! This is really annoying.