Mail Log

Mail Log (/emails/log) answers "what did the mail server do with the mail for this address" — type an address and you get one line per message with its outcome.

Track Delivery (/emails/track) answers the other question people arrive with: "why did this one not get there". It shows a single message's whole route in order, and it has a live view for when you have just pressed send and would rather watch than keep refreshing.

The log is the whole server's. You may only ask about an address in a domain you own, and you only see messages that touched one of your domains.