| Provider | Strictness | ISPs Using |
|---|
| Spamhaus |
10/10 |
IP/Domain blocklists. Used as data source by many. |
| Microsoft SmartScreen |
9/10 |
Used by Outlook/Hotmail. ML-based + reputation. |
| Google Postmaster |
9/10 |
Gmail filtering. Heavy on engagement + reputation. |
| Cloudmark |
8/10 |
Used by major ISPs worldwide. Reputation-based + content fingerprinting. |
| Proofpoint |
8/10 |
Enterprise email security. Advanced threat detection. |
| Yahoo/AOL Filters |
7/10 |
Verizon Media filtering system. |
| Mimecast |
7/10 |
Cloud email security. URL defense focus. |
| Barracuda |
7/10 |
Email security gateway. Strong IP reputation focus. |
| SORBS |
6/10 |
IP blocklists. Dynamic IP focus. |
| SpamAssassin |
5/10 |
Rule-based scoring. Used by many smaller ISPs. |