[LeonM]

If you bring a letter to to post office, you have sent it, but that does not mean it is delivered. It takes the postal service some time before the letter is eventually delivered. during that time the letter is sent, but not delivered. Email works exactly like paper mail.

If there is a security related issue on the receiver side (like an expired certificate), the sender (Mailgun) may hold the email and retry a couple of times. Eventually the issue may get resolved and the email will be delivered, or the sender may choose to deliver the email without encryption (unless MTA-STS is used, of course). During this whole process the email is sent, but not delivered.

I understand that the status 'sent' may be confusing to you, but calling the companies 'trash' just because the email protocol from 1982 does not uphold your standards is a bit harsh.