Echo address, SMTP flow, and the web variant with a one-shot test address.
The classic-mode echo address is echo@tools.onesystems.ch (set by the operator of this installation).
Flow: Send a normal message to echo@tools.onesystems.ch. The service evaluates it at delivery (including SPF, DKIM, DMARC). If email replies are enabled, this service sends the analysis back over SMTP.
Browser alternative: Use create a one-shot test address to get an address like test-…@…. The same checks (SPF, DKIM, DMARC) appear once the message arrives — results are web-only, not as an echo reply.