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.
