Mailgun/bbPress 502 error

We have a bbPress forum setup that has around 700 members that are all notified every time a new topic is started or reply posted.

I’ve recently taken this site on and there’s been a longstanding issue where posting a reply or new topic anywhere in the forum has generated a 502 error. I was under the impression that Mailgun had previously been installed to help resolve this issue but on performing some basic troubleshooting again it seems that the Mailgun plugin is the issue. Turning Mailgun off resolves the issue and it’s possible to generate the error with only Mailgun/bbPress activated.

There’s nothing in the error_log – any ideas as to what might be causing this?

Thanks!