Every recipient server is different and may apply different spam filtering criteria for the messages being received. In most cases, recipient servers do not provide any information about spam filtering to the sender of an email that was filtered. It would give too much information to spammers who would use that data to get around spam filters.
Spam filters are constantly changing to adapt to new techniques and types of spam messages, so what lands in the Inbox today could be spam filtered tomorrow. There are some things you can do to help ensure your emails are being delivered to the inbox.
Most of the major email providers, including Yahoo!, AOL, Hotmail, and Gmail, use engagement-based filtering to help detect spam. This means that the more your recipients interact with your emails by opening, clicking, and reading, the more highly engaged your audience is, and the more likely you'll end up in their inboxes. If a lot of users are marking a message as spam, it's more likely to end up in other people's spam folders. If you've moved emails out of the spam folder, that's a positive indicator and can help ensure future emails that are similar are delivered to the Inbox instead.
Email providers have access to a lot of data they don't share with senders. But if you're tracking opens and clicks, offering unsubscribe options, and using feedback loops (Mandrill handles FBLs for you), you'll get a good amount of data that can help you gauge how your emails are doing engagement-wise. Read more about engagement in the Mailchimp Knowledge Base.
Learn More About Spam Filters
In addition to engagement, most spam filters use some kind of scoring mechanism to figure out how likely it is that an email is spam. If the email exceeds a certain threshold, it's filtered to the spam folder. The threshold changes for every receiving server, and some are configurable by the recipients. Learn more about types of spam filters, and check out the Mailchimp Knowledge Base for information on how spam filters think and tips for avoiding them.