received
When trying to find out why Something Went Wrong during delivery of an email we sometimes want to look at the route by which it was delivered. Did SPF break because of an unexpected forward? Did DKIM break because an intermediate mailserver modified the content of the message? Why did it take nine hours from the mail leaving our ESP to make it to the inbox? Did it really leave our ESP when they say it did? Did Microsoft internal handoffs break something again? Received headers are the breadcrumbs that record the path an email took. Every time a mailserver receives an email it adds a Received header at the top of the mail (so the most recent Received headers come first). And it mustn’t modify any of the existing Received headers. A Received header looks complex, but they have quite a strict structure, and they’re fairly easy to read