I spend a hell of a lot of time from users that use @shaw.ca emails that complain their not getting notifications, can't register (not getting validation email) and it's always been a struggle. Fix one problem, and it stops working again.
With the latest upgrade we moved to a more robust email service, and finally getting some visibility on what's happening. Basically, Shaw is ridiculously aggressive in spam control, to the point where it's fucking over it's users. I talked about it briefly here:
http://forums.beyond.ca/showthread.php?threadid=398677
So what's happening is that we have a lot of Calgary users using @shaw.ca emails, and Shaw's system actively blocks connections from our email server if it detects too much traffic coming from it. I did a small test this morning on a small subset of users, and sure enough, after around 50 emails in an hour, we get blocked. Any emails to @shaw.ca users gets blocked from that point on.
Code:
May 26, 2017 7:51:24 AM
554 mi05.dcs.int.inet cmsmtp Connection refused - 168.245.18.13
May 26, 2017 7:51:24 AM
554 mi07.dcs.int.inet cmsmtp Connection refused - 168.245.18.13
May 26, 2017 7:51:22 AM
554 mi07.dcs.int.inet cmsmtp Connection refused - 168.245.18.13
May 26, 2017 7:51:14 AM
554 mi04.dcs.int.inet cmsmtp Connection refused - 168.245.18.13
May 26, 2017 7:51:06 AM
554 mi06.dcs.int.inet cmsmtp Connection refused - 168.245.18.13
At this point, I can't fix this. We have more than enough @shaw.ca active users where we will send a modest volume of emails to shaw.ca servers, and it just gets blocked. The block lasts hours before it's reset and it just happens again.
So my suggestion is, if you're using @shaw.ca email, switch to something else. I'm going to make a note in our registration page as well that notes this shitty behavior, and recommend not using an @shaw.ca email address.