I’m using the Mailgun API for a couple of clients, making sure we don’t keep sending email to someone who has marked their previous message as spam.

It should be quite simple to use as the docs are very clear, but I kept getting an ‘unknown domain’ error in the returned message when I used the API with one of my client’s domains rather than the sandbox domain the Mailgun provides.

The fix was to use the EU address for the API: api.eu.mailgun.net rather than the standard api.mailgun.net. As my clients are in the UK, they are put into the EU servers, rather than the American ones.

I didn’t find a simple suggestion to do that, so I’m writing this so I find it next time this trips me up.