What it does
Takes a list of domains or email addresses, one per line.
Looks up each domain's MX records in public DNS: the servers that accept email for it.
Names the provider behind them (for example Google Workspace or Microsoft 365), notes security gateways in front of a provider, and flags domains that publish no MX record at all.
Only the domain is looked up. For an address like sam@example.com, the part before the @ is not needed for the check.
Why it is useful in class
DNS is invisible until you query it. The MX lookup shows that every domain publishes, in public, which server receives its mail, and that this record decides where a message goes before anyone reads it.
Ideas for class
Let learners check their school's or university's domain and a few well-known ones, then compare the providers.
Ask why a message to a domain without MX records bounces, and what the sender sees.
Continue with the guide to SPF, DKIM and DMARC, the other DNS records that decide whether email is trusted.
Using this page
The tool is free to use. This description and the ideas on it are licensed under CC BY 4.0.