I remember having some issues like that with a smartapp of mine - it was the TLS version. I believe we default on TLS1.0 which some providers have now disabled. You should be able to get it working by adding the tlsVersion parameter to the request Map and setting it to TLSv1.1 or TLSv1.2 as seen below:
return httpPost([
uri: [url goes here],
tlsVersion: "TLSv1.1",
])
Please let me know if that fixed your problem.