There could be many reasons why your messages are going straight into recipient spam boxes. SimplyCast maintains relationships with ISPs and we also monitor our systems to try and avoid deliverability problems; however, all this will not prevent your message from ending up in those folders if your recipients' spam filters are triggered by your message (such as the subject line used, message content, etc).
We recommend that you check your campaign's content and subject line with a spam checking tool and modify the content to correct any results that are found. Avoid using words and phrases such as "Free", "Click Here", huge fonts, and images that are too large.
We also recommend that you send to a good list that is free of bad email addresses. If you send out a campaign and receive a lot of bounced mail (1% or greater), this can affect the acceptance rate of your emails by the ISPs you are trying to deliver the messages to because they may decide to block the content that you are sending.
The SimplyCast Blog contains many useful articles about how to keep your emails compliant and improve your deliverability.