SpamExperts uses a lot of different technologies to decide whether a specific email should be seen as spam or not.There are a few stages how it performs filtering: pre-data, post-data and a stage in between these two.
The pre-data stage includes Connection rate limiting, IP Blacklist checking, destination domain verification, SMTP protocol compliance, sender address inspection, recipient validation checks, whitelist/blacklist of sender and/or recipients, recipient connection rate limit, SPF checks, content reputation feeds, bounce address tag validation as well as simple private signatures.
On the post-data stage, this includes validation of DKIM and DMARC, rules defined by a specific user, virus scanning, DNS and URI reputation checks, fingerprinting, statistical information about senders and heuristics defined by SpamExperts.
In between these two stages, there's also a direct feed looking at general IP reputation, if the sending IP is known for spending spam, and their sending history.
With most or all these metrics - the software will calculate a combined score - and depending on this score it will either accept the email or mark it as spam and thus putting it in the quarantine.
Some of the pre-data checks will result in dropping the emails before even reaching the post-data stage, this only happens if the sending server is poorly configured or being a part of one or more blacklists.