As mentioned above, ISP Spam Filters utilize a variety of tools and processes to identify incoming emails as possible spam.
They look for spam-esque characteristics, calculate the likelihood of the email being malicious or spam, and then weigh the total risk against a predetermined threshold. If it is too high, the email is flagged as spam.
Typically, the tools analyze:
Email headers – focusing on suspicious email addresses or ones that have not gone through
a checklist for deliverability, or aspects such as misspellings.
Email content – Filters will check for file names, bolded or italicized or all caps, text size, images, and attachments.
Blacklist filters – they will compare IP addresses and domain names to those of senders on existing blacklists, as they can cross-examine.
Utilizing tools to warm up and determine if you’re on a blacklist is very important when setting up a new domain.
Permissions – filters will compare to what the recipient has previously agreed to receive from the sender. Anything that has not been agreed upon will go to spam.
Rule-based filtering – these are the filters set up by algorithms to identify spam, such as particular phrases. There are particular words or phrases that spam checkers will help you recognize and avoid.