It's more or less same as mailstreet et al... you can have the whole list of services that bell offers including hosted exchange/bes. I don't know specifics about port blocking, nor do I know pricing. I also know Rogers has thought about this for sometime but don't currently have the manpower to do this. This is not to say they cannot do it, but they apparently don't hire an entire dept. to offer a new service such as this.
I spoke with someone today at Bell about their BES service. Explicitly I want MDS, and just like everyone else you need to buy a base package and then an additional. So the total cost comes out to be CDN$35/mth. I used to pay myblueberry U$10/mth but got fed up with them. I'm hesitant to pay Bell the extra $35 but on the other hand sparring with Bell is always a fun game of chess.