Someone (I think Shawn) was asking about this last meeting. I just hit this problem (or one related) also. Looks like MTS mail servers are not accepting email from RBL'd (ie: all Shaw dynamic IP's) servers anymore. If you run your own mail server on Shaw and you don't smarthost all email, you'll have to add a rule just for MTS to your /etc/mail/mailertable like:
mts.net esmtp:shawmail.wp.shawcable.net
and more for any mts-related domains (that all use the same MX) like escape.ca.
Note, this won't apply to most people with simple email setups: this only applies to us weird people trying to run mail servers off of dynamic IP's.
And, yes, leave it to MTS to do this in a non-standard way: they are just dropping the connection giving 554 service unavailable when they should be giving us a bounce with an indication we're being RBL'd so we know what the heck is going on.