Every time a message is received at Microsoft side, it is going through their spam filtering system which gives it a spam score. The related spam score will then be assigned an individual Spam Confidence Level (SCL) rating which is then included as an X-header to the related email header.
Microsoft automated systems will then decide what to do with the related message depending of the Spam Confidence Level rating assigned.
The following table shows the actions taken depending of the SCL rating:
| SCL Rating | Spam Confidence Interpretation | Default Action |
| -1 | Non-spam coming from a safe sender, safe recipient, or safe listed IP address (trusted partner) | Deliver the message to the recipients’ inbox. |
| 0, 1 | Non-spam because the message was scanned and determined to be clean | Deliver the message to the recipients’ inbox. |
| 5, 6 | Spam | Deliver the message to the recipients’ Junk Email folder. |
| 7, 8, 9 | High confidence spam | Deliver the message to the recipients’ Junk Email folder. |
Source can be found here