replace a dead tool by another one

docs/prerequesite-dns.md

@@ -71,7 +71,7 @@ Here are some tools you can use to verify your DNS configuration:
 
 - [MX Toolbox](https://mxtoolbox.com/SuperTool.aspx) (DNS, SMTP, RBL)
 - [port25.com](https://www.port25.com/dkim-wizard/) (DKIM, SPF)
-- [HAD Pilot](https://www.had-pilot.com/testdetails.html) (DKIM, DMARC, SPF)
+- [Mail-tester](https://www.mail-tester.com/) (DKIM, DMARC, SPF)
 - [DMARC Analyzer](https://www.dmarcanalyzer.com/spf-record-check/) (DMARC, SPF)
 
 ## Misc