Developer sandbox
Point your application's outbound mail at a sandbox address and every message it sends is captured here instead of going anywhere near a real person. Useful when you are testing a signup flow, a password reset, or a nightly job that emails a thousand people.
No account, no card. You get a link that is the only way back in, so keep it somewhere your team can find it.
Why not just use a throwaway inbox
A throwaway inbox is one address that expires within the hour. A sandbox is a catch-all that lasts a year: anything after a + lands in the same place, so yours+alice@etymail.com and yours+bob@etymail.com both arrive without creating anything first. That is what a test suite needs.
What you can see
The subject and body, the raw .eml, and the SPF, DKIM and DMARC results your message actually earned on the way in. If your app's mail fails authentication here, it will fail at Gmail too, which is the point of looking.