Different types of spam filters
Despite the fact, that the global spam rate is decreasing, in 2020 47% of all sent emails were considered as spam. Different types of spam filters protect users from spam emails.
If the sender is known for bad mails, the mail is rejected. Spammy content goes into the spam folder. Mails which had, for example, been read by others who received (mass mailings) them or which look like personal mails go into the inbox. If mails of a sendout to several users are marked as spam by some users, then the rest of the sendout is going to spam automatically.
A combination of the following techniques are used to rate emails.
Rating by IP
Every email has to be transmitted into the internet via an IP. Email receivers (like Google, Outlook, GMX) rate the IPs from which they receive mails by the quality and quantity of the mail they send. If an IP is known for spam, mails get blocked or are sent to the spam folder. There are also blacklists which have a database of known IPs which send spam. So if one email service recognizes heavy spamming from an IP and reports the address, it is likely, that others services which use the blacklist block that IP, too.
Rating by domain
The domain of the email sender (like email-knowledge.com in firstname.lastname@example.org) is rated, too. It works kind of like the IP rating.
Content of emails
Of course the content of emails is examined by the email services. The filters search for specific words and patterns which often occure in spam emails. Then they rate the email. An email with a bad score is not accepted. Not to forget is the check of correct HTML and sometimes even the images are scanned.
As mentioned earlier the user engagement (opening or moving a mail to spam, for example) are recorded and the messages of a given bulk mailing are treated the same way the users reacted to it. Example: If the first 100 receivers marked the mail as important, then the mail may go to the inbox of all receivers.
A trick is used by ESPs (email service providers) to find spam senders. They slip spam traps (email accounts just used to check what is received) into the lists of spammers. If a spam mail is found in the spam trap, they block the sender.