How does email service provider filter spam messages?

Every service provider has their own set of rules and regulations and anything that goes against the laws is treated as spam. Here is how the different service providers handle the spam filtering -
Gmail
Gmail depends on its users and community to determine whether a email is spam or not. If any of your subscribers treat you As spam, your address will remain recorded as spam. Also having your content similar to what has been marked as spam, will be treated as spam.
Your IP address and domain also plays a role in determining whether your email ends up in spam folder.

Yahoo
Yahoo invariably scans the IP address and domain . It scans the actions taken by the user's in connection with your mails. If lot of users are found to delete your mails without even checking it or send it to a certain folder, your subsequent mails are treated as spammy.

There are some common rules that these service providers follow. They keep scanning the way it's subscribers are interacting with the mails.

Most of the email providers have a method of detecting spam messages due to usage of spam words. So once the word within subject or message is triggered then the delivery of that message goes into junk or spam. This in turn reduces the spam delivery. Spam is also sent from the set of IP addresses. So those sets if blocked then delivery of the spam is reduced.