Mail server checking tools

The best - http://www.mail-tester.com

https://mxtoolbox.com

https://testconnectivity.microsoft.com/

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

http://dnsstuff.hostpro.ua

SPF https://vamsoft.com/support/tools/spf-policy-tester

smtp-checking-swaks-openssl-telnet