If the spam score for one of your messages is 10 or higher, you probably have a URL in your message that is on a blacklist.
Blacklists are usually run by independent groups who attempt to monitor malicious activity such as spamming, phishing, and so on. They often have connections with ISPs like Gmail, which means that being on a blacklist can prevent your messages from making it to subscribers' inboxes.
What Should I Do?
To solve this problem, click on the spam score. Here, under the Score Breakdown header, you should see an entry that looks like this:
Click the link in that entry, and you'll be taken to a page where you can search the blacklist that is affecting your message. Search for any URLs/links that you've included in your message in order to identify the problem.
Once you've found the problem link, you can either remove it or contact the blacklist in question and see if you can work together to remove your URL from their list.