Vidéotron do have some spam filtering...
problably the mailing list is sending email from an known spamming host or is badly configured.
also when there are too many recipients in the email, it will be automatically be blocked by some server (again to protect against spam).
You might want to contact technical support for that...
I doubt that anything could be done... but maybe they will be able to somewhat whitelist the mailing list (if coming from a mailing list) to let the emails go through