If Telus didn't have a misconfigured SPF record that caused host_check() to inadvertently (always?) return "pass", Gmail likely would have classified the messages as spam and the spammers probably would've quickly gave up.
On the other hand, one could reasonably argue that Gmail -- with all of their advanced algorithms and such -- should have been able to easily determine that these messages were "forged" (the absence of a valid DKIM-Signature: should have been a giant red flag, for example) and reject them, ideally, at SMTP time or, at minimum, immediately dump them into a user's folder. Likewise, a more restrictive DMARC policy (i.e., "p=reject") than what Gmail currently has ("p=none") would also have caused these messages to be rejected (although DMARC can potentially introduce other issues or unintended side effects).
Alternatively, you could blame it on the spammers.
