Modern spam filters use different signals to classify an email. How exactly it is done differs by implementation, but in general the sender only has indirect control over many of those. Just because an email system is cleanly implemented, does not mean it won't be sending spam; using that as the only signal would defeat the purpose.
- SPF and DKIM ensure that the email was emitted by the owning system, preventing spammers from abusing random addresses.
- A valid rDNS suggests that you actually have some control over the IP you're sending from.
- Then there are blacklists, which some systems, e.g. SpamAssassin, query to identify known bad senders.
- Large email services often keep statistics on a sender through their own users feedback. If enough people hit the "spam button" on the first messages of a campaign, it's likely to end up in the spam folder for everyone else.
- Some mail providers share IP reputation data with third party services, which in return provide a more detailed analysis of a sender.
- Content analysis also plays a role. If an individual message contains questionable content (think Viagra), it may get classified as spam despite originating from a reputable sender.
- Sending large amounts of mail to non-existing addresses, i.e. using dirty lists, is another hallmark of spammers and has a negative impact on reputation, too.
In a nutshell, being trusted by the major providers like GMail, Yahoo or MSN takes time and consistent sending of quality content that recipients actually want, and won't get flagged by users or content analysis.
If you use an address from a VPS hoster, where IPs are a dime a dozen and change ownership often, you'll start out with rather low trust. Use reputation and blacklist checks to ensure the IP doesn't already have a bad history. If it does, contact the list maintainers where possible, and explain that the IP is under new ownership. Most will allow a fresh start.
If you send bulk email, you can also participate in feedback loops to remove people who classify your messages as spam from your list, and as such avoid the associated penalties in the future.