Anti-Spam blade does not support any kind of e-mail encryption (STARTTLS, SSL/TLS).
- E-mails sent to public e-mail providers such as Yahoo, Gmail, etc. via web browsers are sent over HTTPS.
- E-mails sent from public e-mail providers such as Yahoo, Gmail, etc. to your organization e-mail servers are sent over SMTP.
Plain SMTP should be detected by Anti-Spam (if the topology / IP reputation is configured correctly).
Depending on your e-mail servers configuration, most of SMTP is not plain (for security reasons), but some sort of encrypted SMTP (for example, SMTP/TLS).