Update SpfCheck.cs

Check if the domain just has one spf record. See: https://datatracker.ietf.org/doc/html/rfc4408#section-3.1.2

BusinessMonitor.MailTools/Spf/SpfCheck.cs

@@ -79,6 +79,11 @@ private SpfRecord GetRecord(string domain)
                 throw new SpfNotFoundException("No SPF record found on domain");
             }
 
+            if (record.Length > 1)
+            {
+                throw new SpfNotFoundException("Too many SPF records found on domain");
+            }
+
             // Parse and validate the record and return it
             var parsed = ParseSpfRecord(record);