Merge pull request #2564 from drwetter/starttls_phrasing

Phrasing of reason for STARTTLS grading improved

Author: drwetter

testssl.sh

@@ -22933,7 +22933,15 @@ run_rating() {
      pr_headlineln " Rating (experimental) "
      outln
 
-     [[ -n "$STARTTLS_PROTOCOL" ]] && set_grade_cap "T" "Encryption via STARTTLS is not mandatory (opportunistic)."
+     [[ -n "$STARTTLS_PROTOCOL" ]] && set_grade_cap "T" "STARTTLS encryption is not mandatory for clients. STARTTLS can only be secured client-side"
+
+     # TL;DR: E-mail transfer via port 25 is broken and the amendments suggested so far are duct tape. So please do not expect testssl.sh to shut up.
+
+     # Explanation: For other than SMTP you should use TLS as per RFC 8314 . For SMTP however there's this thing named reality: A mail server cannot
+     # just switch to the mail submission port 587 only and continue to receive mail from everyone. Even if you advertise this via SRV record (RFC 6186).
+     # For STARTTLS there's no way to tell for testssl.sh whether it is secure. A MitM can always intercept the connection, unless the client checks
+     # the certificate accordingly (it's getting better but some just don't). TLSA Records/DANE and MTA-STS (RFC-8461) on the server side can help too.
+     # But as said, it's useless unless the client MTA checks all that which no tool can check.
 
      pr_bold " Rating specs"; out " (not complete)  "; outln "SSL Labs's 'SSL Server Rating Guide' (version 2009q from 2020-01-30)"
      pr_bold " Specification documentation  "; pr_url "https://github.com/ssllabs/research/wiki/SSL-Server-Rating-Guide"