I am having the same problem with my Scrabble game provider in the UK. Shaw acknowledged that they have an issue and are "working on it." Don't go changing your settings. Wait for Shaw to do their jobs.
It seems Shaw has tightened the security and filtering on email around the beginning of June. I had some automated scripts that sent emails via smtp.shaw.ca. I just noticed one script stopped working on June 12. The previous night, it was working. I had to adjust the script to get it working again. In particular I to change the subject. It would reject an email with a subject of "An Account Failed To Log On".
@Glindgren -- ... reject an Email with ...
Weird.
Sending from Shaw WebMail to my GMail ID does not arrive at my GMail ID.
Sending from my GMail ID to my Shaw ID does arrive.
Sending from another ISP to my Shaw ID does arrive.
Perhaps, that specific SUBJECT-line is 100% blocked, for everybody?
@Glindgren -- my experiment's results, when tweaking the SUBJECT line:
Hmm.