fix rapid7#8305, escape unadorned periods in the front of SMTP payloads

Author: Brent Cook

lib/msf/core/exploit/smtp_deliver.rb

@@ -194,6 +194,8 @@ def send_message(data)
         full_msg << date unless data =~ /date: /i
         full_msg << subject unless subject.nil? || data =~ /subject: /i
         full_msg << data
+        # Escape leading dots in the mail messages so there are no false EOF
+        full_msg.gsub!(/(?m)^\./, '..')
         send_status = raw_send_recv("#{full_msg}\r\n.\r\n", nsock)
       end
     else