Why cannot I see my test email in the inbox or spam box?

Most of the time, it is because your domain already has an SPF record. Our email was sent using the shared domain of our app, with the from sender is your domain. But if the sender domain (from domain) has a strict SPF like v=spf1 include:_spf-eu.ionos.com ~all then our email would be unauthenticated. You can go to MX toolbox (opens in a new tab), use the SPF tool to check your domain's SPF record:

In order to allow our app to send the email on your behalf, please add our domain mailgun.org to your existing SPF record, like: v=spf1 include:_spf-eu.ionos.com include:mailgun.org ~all

Note:

Remember to add to your existing SPF record, not adding another. Multiple SPF records will cause you trouble instead.