Sandbox subdomains are for test purposes only. Please add your own domain or add the address to recipients in Account Settings

in 2019 is:

Click to your domain, and you see at the right the Authorized Recepients box

i have faced this problems.

if testing with sandbox, should be add the authorized recipients. then copy the sandbox name as a domain.

Thanks you.

Log in to your Mailgun account, go to the log section. There you should see a log entry for the failed attempt to delivery that email. Click on it to see all details. it should display the full error message as well as additional information.

However, most likely you forgot to add that specific email address to the list of authorized email addresses.

Side note: after adding yourself to authorized recipients, you also need to verify your email address. I almost disregarded the following email as commercial, but in fact I kept getting the error until I agreed:

Hi there,

Mailgun account "[...]" provided your address to test their integration with Mailgun.

Please click the link below if you agree to receive emails from their account.

I had the same problem I solved this by doing below steps.

1. Add an email to Authorized Recipients - Steps.
2. When you add from and to emails Don't add Quotation marks