A successful send request does not establish inbox placement. For the Mailgun entry, the event definitions distinguish accepted, meaning queued, from delivered, meaning accepted by the recipient's mail server.
Neither event proves that a person read the message. Model those milestones separately and retain the message identifier so later failures or delivery events update the right record.
Test with consenting recipients and inspect the event trail rather than repeatedly sending the same email when its outcome is uncertain. Keep addresses and message bodies out of public debugging examples.
Akousa