I have been having a hell of a time sending email via authenticated SMTP when not on a Shaw internet connection. The problem has been intermittent for months but has recently gotten way worse.

I haven't been able to send an email from either my phone (iPhone, iOS 15, Mail app) or laptop (Mac OS 10.15, Mail app) in days.

This evening I turned on logging in my Mac email client, and I'm seeing the following. I highlighted the concerning bit:

READ Jan 07 20:44:59.071 [kCFStreamSocketSecurityLevelNone] -- host:mail.shaw.ca -- port:587 -- socket:0x6000037bc120 -- thread:0x6000044adb80
421 shw-obgw-4002a.ext.cloudfilter.net cmsmtp 172.56.42.152 blocked. AUP#DNS

I am not running any email servers or servers of any sort. I definitely do not have any malware running within my home - I run a tight ship. I do not send marketing email campaigns or anything like that. This is my personal email only, and I don't send a high volume of email whatsoever.

The ISP I use when not on my home Shaw connection is, I believe, using IPv6. This is a little out of my area of understanding, but is it possible that all IPv6 addresses from this ISP are getting caught up in an overly broad blockage from Cloudfilter (a provider contracted by Shaw)?

Shaw technical folks, please, this is driving me insane. I cannot use webmail full time. Believe me when I say there is no legitimate reason for me to be blocked sending email from my personal email client by Cloudfilter. Please support your customers and escalate with Cloudfilter.