Do you ever thought why do the mails sometimes lands up in inbox and some in spam or junk ?
Whether your email is marked as spam?
The sender reputation or the IP reputation are the most important factors that comes into picture when determining will your email land in inbox or in spam folder and the IP address that the sender reputation depends upon the IP that it is bound to.
What ISP does is they assign a particular score to the IP address based on varied factors like spamming, email marketing, etc., and then based on their parameters and rules, they determine the IP reputation. This reputation build the trust to the source of the email. This trust varies from ISP to ISP.
There are number of factors the ISP looks in before determining the score. A reputation, be of any kind, does not build overnight. Building a reputation takes time.
In my professional experience, I have seen many client's complaining of mails being marked as spam even when the mail server IP is brand new IP, " We have brand new IPs, and we our mails are still going to Spam, how is it possible ? " Well, see it like this, IP with no reputation is also suspicious.
Example 1): A person with no background is always suspicious. You will never give your house on rent to a person with no background, unless a police verification is done. This verification is the reputation in terms of ISP. If they do not know any history about the IP, it will be marked as suspicious.
Example 2): A bank will give you loan easily if you have a good reputation of credits and on-time payment history and will not even entertain you if do not have a good reputation.
The same goes with the IP reputation. An IP with good reputation is welcomed and IP with bad reputation is not.
What the factors that you should be aware of?
1. How many users mark your mails as spam and how many of them complains this to the ISP about this email. Each and every activity is measured and considered for Marking IP reputation.
2. Having valid email list: If you are an email marketer, how many email address you have are valid ones. The more the mails go to invalid email account, more will be the bounce mail and will be considered in marking the reputation score to the IP.
3. How many emails you are sending from the mail server. Volume of mails are considered, as this will directly be checked for bounce too.
4. Apart from their marking parameters, they also check the other external blacklists to check for IP reputation, so ISP will have their verification parameters as well as they will check it from external source too.
5. Valid DNS is the important aspect of reputation too, especially Reverse DNS.
6. Last but not the least, will be the email content, spam contents in it, links or URL that will cause issues to the recipient like malicious links and all those stuffs. Reasons: Infection on the server can cause it to send spam. It is not necessary that you are involved in such activity, on a shared server, there could be another user who has done this resulting in mail server IP being caught in the blacklist. In such case, hosting provider is the one who is responsible for doing all investigation.
One other possibility is that someone who is in your IP range network is sending spam and the Blacklisters decide to block the complete IP Range and in an attempt your IP address being caught in the IP range listed. In such cases, you will find your very much helpless.