There is definitely a problem going on with email between Telus accounts and Shaw accounts which started around May 1. Emails are intermittently lost. I setup a test Telus Webmail to Shaw Webmail (so there are no email client issues to be concerned about). Since two of my neighbors use Shaw I have both Telus Webmail and Shaw Webmail open and running on my PC.
I generate five test emails: Test 420, Test 440, Test 460, Test 480, Test 500.
One arrives at Shaw fairly quickly
One arrives after a long delay of more than one hour
Three never arrive at all. Even after a wait until the next day.
EMail to both my neighbours with Shaw is hit and miss these days. Since around May 1 they tell me they do not receive all my EMails. I can sit in their kitchen and send them two emails from my tablet. They may get one, they may get two or they may get none.
(I should mention that all five show in Telus Webmail Sent Mail folder.)