If your email marketing messages are landing in SPAM or being rejected by SPAM filters, you are probably using a free email provider like GMAIL or Yahoo. If your email ends with @gmail.com, @aol.com, @yahoo.com, @att.net, @hotmail.com, @(any domain you or your company don't own), you are not going to be able to send email marketing with any continued success.
If you are using your own domain, and your clients are not receiving mail from your CORE e-marketing campaigns, then check your SPF settings.
If you don't own your domain or your company doesn't control the domain, then you won't be able to add the SPF record you need to use CORE. Email providers are working hard to eliminate SPAM from their users' inboxes, so use this setting to determine if mail should be delivered or not. If you don't have that setting in the domain you use for email, please purchase your own domain and email and use a custom email address so you can make this change.