Domain-level checks tell you the domain is set up correctly. They do not tell you that one specific mailbox on it is the problem — wrong authentication, sending too fast, or already sitting in a filter's bad books.
Every mailbox now has a health check producing a single verdict, with the individual checks behind it listed. Run it on demand, or read the last result from the mailbox list.
Failing checks come with an explanation of what failed and what to do about it, in a drawer rather than a tooltip, because "SPF alignment failed" needs more than eight words.