Comprehensive DNS and email testing tools

https://mxtoolbox.com/

Check MX record configuration

https://toolbox.googleapps.com/apps/checkmx/

Global DNS propagation checker

https://dnschecker.org/

DMARC implementation guide

https://dmarc.org/overview/

Test and validate SPF records

https://www.spf-record.com/

DKIM implementation resources

https://dkim.org/

Official ICANN domain lookup

https://lookup.icann.org/

SSL/TLS configuration testing

https://www.ssllabs.com/ssltest/