https://www.openspf.org/FAQ/Common_mistakes
Online IT tools, checkers and info
DNS settings:
https://mxtoolbox.com
https://dnscheck.pingdom.com
https://dnschecker.org <— DMARC, SPF
Mail server check:
https://www.mail-tester.com/
http://www.uceprotect.net/en/rblcheck.php
DNS Server + its settings
IP Lookup:
http://www.whois-service.ru/lookup/
https://whatismyipaddress.com/ip-lookup
https://ipinfo.io/ <— also with ASN lookup
Domains: info, subdomains, IPs, history:
https://subdomainfinder.c99.nl/ <— Make sure to check “Private Scan”
https://securitytrails.com/
Current IP:
https://myip.com
https://myip.ru
https://ifconfig.me/
https://checkip.dyndns.org/
Crontab:
https://crontab.guru/every-1-minute
Check crontab execution: grep CRON /var/log/syslog
MAC address Vendor Lookup:
https://www.macvendorlookup.com/
https://macvendors.com/
Unix Timestamp:
https://www.unixtimestamp.com/
Free tool to unminify (unpack, deobfuscate) JavaScript, CSS, HTML, XML and JSON code, making it readable and pretty.
Online Python:
https://www.online-python.com/
Temporary mails:
https://temp-mail.org/
https://www.mailinator.com/
10minutemail