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Help Center / How does Spam Filtering work?

When a sender’s Mail Server attempts to connect to your Mail Server to deliver email, we first check the sender’s Mail Server against our Blacklist of known abusers (Brute-Force Hackers, Email Harvesters, and High Volume Spammers) . If they are listed then the connection is dropped as if your Mail Server does not exist. We routinely block @ 8 million connections every day from a list of  127,000-500,000 known abusers that are continuously updated.

If they pass this primary test, we then say “HELLO” to the sender’s Mail Server and ask them to identify themselves and who they are sending mail for. If the identity of the sender’s Mail Server is not a FQDN (Fully Qualified Domain Name) or cannot be verified through a rDNS (Reverse Domain Name System) lookup then their email is not accepted for delivery. However, if the identity of the sender’s Mail Server checks out as valid then the sender’s address is also checked against a list of known Spam domains and email addresses. If the sender is listed then their email is not accepted for delivery. If they fail either verification then the sender should receive a NDR (Non-Deliverable Response) from their Mail Server telling them that the message was undeliverable due to Spam Filters. If, on the other hand, either the sender’s email address or their Mail Server is recognized as “Trusted Sender” then the message goes straight to your Inbox.

If the sender’s email address seems okay but your Mail Server does not recognize the sender as either a Trusted Sender or as having sent email to you in the past 288 days (10 months), then we ask the sender’s Mail Server to please try again in 120 seconds. This waiting period is called Greylisting and is really effective at stopping Spam because  most spammers do not attempt redelivery, whereas legitimate Mail Servers are required to attempt redelivery a minimum of 4 times. A two minute waiting period for an email from an unrecognized sender might seem to be an inconvenience but Greylisting can be 90% effective at reducing Spam as spammers have more than plenty Junk E-Mail to deliver elsewhere to bother returning 120 seconds later to deliver just to you.

Once the message passes Greylisting, or is recognized by your Mail Server for having sent email to you in the past 288 days, then the message is subjected to 179 tests to determine if it is Spam (Junk E-Mail). Each test assigns the message a score, ranging from a maximum of -10 for each HAM (the opposite of Spam) and Whitelist checks it passes to 1-20 (with an average of 5-10) for each RBL (Realtime Block List), URIBL (Uniform Resource Identifier Block List), Checksum, Content Filter, or Bayesian/Statistical check it fails. After all of these tests are run the total score is tallied and an action is determined based on a scale of -100 to +100 as follows:


Total Score Threat Level Default Action
-100 to 9 Good Deliver to Inbox
10 to 19 Low Prefix Subject with “Junk EMail:”. Deliver to Inbox
20 to 29 Medium Quarantine in Junk EMail Folder
30 to 49 High Quarantine in Junk EMail Folder
50 to 100 Severe Reject Message/Bounce back to Sender

By default most email clients (Outlook, Windows Live Essentials, Apple Mail, Thunderbird, etc.) do not download the contents of their Junk EMail Folder and this folder is normally accessible only through your webmail account. If your email client is using IMAP or EWS to synchronize with your email account you should be able to manually subscribe to this folder if it is not already subscribed.

If desired, you can also override the way that your Mail Server handles Low, Medium, and High probability of Spam messages by logging in to your webmail account at  and going to SETTINGS > FILTERING > SPAM FILTERING. You can choose what actions you want taken for these three levels. Your options are NO ACTION, MOVE TO FOLDER, ADD TEXT TO SUBJECT, or DELETE.


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