Cannot send email via SMTP. Appears to be Shaw's Cloudfilter blocking

[Matt_c]

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. 

[yohan4ws]

I've been having the same thing since July this past year.  The ISP (another Canadian trusted ISP) doesn't have reverse DNS on their IP blocks.  No one at SHAW support understands what's going on, or cares enough to do something about it. 

Grasshopper, You should try and adjust your IP settings if on a computer and disable IP v6, not sure if this can be done on a phone .. that MIGHT work ?  But if there is no reverse DNS or PTR record on the public IP you're using for internet, SHAW Mail will not allow you to send. 

I feel your pain ... I'm an IT Network Administrator,  this isn't port 25, from outside SHAW networks you can't connect on port 578 to relay email anyways, the connection is SSL Secured encrypted and requires password authentication ... This additional provision doesn't make any sense if they geo filter IP's to Canada at the very least to bypass this.  Cloudfilter has that ability.

Further, recently I had a problem e-mailing @shaw customers from my business email on m365.  I used an outgoing phishing filter that uses AWS, Proofpoint which SHAW uses to block phishing was set to block all AWS services  I was hours and 3 attempts with shaw to resolve this, I finally e-mailed proofpoint directly and they resolved it by whitelisting my domain.  Maybe try and reach out to cloudfilter directly ?  I haven't had time to.

[srglassw]

Disabling IP v6 resolved the issue for me. Looks like when I upgraded from Windows 10 to Windows 11 IP v6 was enabled by default.