Hi kenshin737,
Since you're receiving some emails, your email server is not blacklisted but spam filter doesn't work like that. It works on "points" (more or less, the reality is that they use Bayes theory, but for this example it's better to think in points).
Let's say, you have several key parts to score in an email (from 0 to 10 each):
- Server's ip
- Server's address (
addres@yourdomain.com)
- Keywords in subject or body (words such as buy, shop, viagra, enlarger your penis, ... etc)
- If it has links in it or not.
- "Template/shape" of the email (if it's only text, or only an image attached, ...)
- If it has attachment
- If something known as bad is in the content (such as a link to a known malware website).
- ...
If the spam filter is a good one, it could take more consideration, for example, the language of the email. (It's strange that if all your previous emails are in english, you suddenly received an email in spanish).
Each key point is rated from 0 to 10.
For example, is the IP is from Russia or China, it probably get scored high, maybe a 7 or higher.
If the word "viagra" is in the email, then you add another 10 points
If it has a link to a shop, add another 10, but if the link is to a website then 5 for example.
...
Later the filter sums everything (ip score+ word score + links score + ....) and if the score is too high, then it's marked as spam.
For example, you could send an email to a friend and pass him a link to a funny website, it will be delivered. But if instead, you pass him a link to a to to buy viagra an the subject of the email is "BUY VIAGRA", you'll probably be tagged as spam.
All this being said, your server's ip might have a high score as a spam ip (even if it's not blacklisted), since you're un a VPS, means that the same IP is shared with other people. Shared IPs/servers are not good as email servers. You should ask your hosting company/internet/forums help with the configuration of your email server. Maybe there's something it's not correctly configured (for example, if you have yourdomain.com and yourdomain.net, and are mixing up both domains). In some cases, you need to validate your server to enter a "whitelist".