Dylan Sather (Pipedream) : I think this doc may help re: troubleshooting the 400 error: FAQ: How do I troubleshoot 400 errors?.
This is not an issue with Pipedream. Take a look at the How do I get more information on 400 errors? section. That’ll show you how to see the error data Mailmodo is returning, which should help you troubleshoot the issue. If they don’t return that data or it’s not clear what the issue is, I’d recommend reaching out to their team to show them the data you’re sending in that request, and see if they can help you understand the 400 error more.