| `mailbox_confirmed` | valid | Remote mail server confirmed mailbox exists with 250 OK | 1 Credit |
| `mailbox_not_found` | invalid | Remote mail server returned 550 User Unknown | 1 Credit |
| `failed_syntax_check` | invalid | Malformed address failing RFC 5322 standard | 1 Credit |
| `no_dns_entries` | invalid | Domain does not exist in authoritative DNS | 1 Credit |
| `no_mx_records` | invalid | Domain has no Mail Exchanger (MX) records | 1 Credit |
| `disposable` | disposable | Matches 100,000+ burner domain firewall | 1 Credit |
| `role_based` | risky | Departmental inbox (e.g. info@, sales@) | 1 Credit |
| `accept_all` | catch_all | Domain accepts all addresses blindly | 1 Credit |
| `antispam_system` | unknown | Remote MTA blocked probe or requires human challenge | 0 Credits (Free) |
| `failed_smtp_connection` | unknown | TCP port 25 connection timed out or dropped | 0 Credits (Free) |
| `greylisting` | unknown | Remote MTA deferred connection per greylisting policy | 0 Credits (Free) |