Different mail providers have different settings for junk mail thresholds. There are a couple of things you can do to help this, make sure that the subject of the email is informative and correct for the context of the message. You can also check on the from name and address, if the from address matches the domain it was sent from it should give you a better result. Also, these filters will often be triggered on the content of the message, so make sure that you have only relevant info in the message and avoid using external links.