emails failing with 554 from Mailgun SMTP

host smtp.mailgun.org[104.130.177.23] said: 554 Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authorized recipients in domain settings.

Mailgun says you are using it in the wrong way.

Well, yes, now I come to discover we use Mailgun on our server rather than having it on WordPress. Argh. Sorry for bugging you and thanks for the quick response.

I started getting the exact same error on Dec 1st as well. I changed nothing and eventually found my log files on my local mail server that game me this exact same 554 Sandbox error.
Did you figure it out?

I found this:
https://help.mailgun.com/hc/en-us/articles/217531258-Authorized-Recipients

Not sure why Dec 1st it broke for me but I added one of my recipient accounts and will see if that fixes the problem after sometime since my immediate test failed again but will try again later.

Actually, I did figure it out. We’re hosted on Rackspace and Mailgun is their product. I’m not sure if they just automatically use Mailgun if you want to do mail through them, but that’s what we had (to be perfectly clear, they use an MTA on the server, but use Mailgun as a relay host). Apparently, we set it up to use the non-production sandbox mode because it has tighter restrictions on quantity of mail, which we were using to prevent the case of some web spider ignoring our robots.txt and trawling our custom product builder, which could generate lots of spurious transactional emails.

The problem lies mostly here, as they made changes to domain verification that required us to add a sending domain (note, however, that Rackspace never alerted us to this fact). Your situation may not be the same, but should be related. Note that only the Rackspace account owner can access the Mailgun settings, which can find instructions about here. Good luck.