No, I'm not joking. I guess I'm just really dense. I've also tried passing -L 100 in addition to -I, but the result is the same.

Why are you using xargs to read to an entire entry? Why not set it as a var and place it in the comment or take it as stdin?

I've also tried this:

   msg=$(cat message)
   curl -X POST "http://sa-api.postmarkapp.com/filter" \
    -H "Accept: application/json" \
    -H "Content-Type: application/json" \
    -v \
    -d '{"email":"'"$msg"'", "options":"long"}'

But sa-api.postmarkapp.com returns "error"... :-(

oh no. I'm sorry! that's the previous url which we moved last minute.

I'm going to go through the doc again to make sure there's nothing else out of place :/

So I see you've updated the doc with a new hostname, but I still get an error, so there's something obviously wrong with my method:

  msg=$(cat message)
  curl -X POST "http://spamcheck.postmarkapp.com/filter" \
    -H "Accept: application/json" \
    -H "Content-Type: application/json" \
    -v \
    -d '{"email":"'"$msg"'", "options":"long"}'

  * About to connect() to spamcheck.postmarkapp.com port 80 (#0)
  *   Trying 184.73.163.218... connected
  * Connected to spamcheck.postmarkapp.com (184.73.163.218) port 80 (#0)
  > POST /filter HTTP/1.1
  > User-Agent: curl/7.21.7 (x86_64-pc-linux-gnu) libcurl/7.21.7 OpenSSL/1.0.0e zlib/1.2.3.4 libidn/1.22 libssh2/1.2.8 librtmp/2.3
  > Host: spamcheck.postmarkapp.com
  > Accept: application/json
  > Content-Type: application/json
  > Content-Length: 5229
  > Expect: 100-continue
  > 
  < HTTP/1.1 100 Continue
  < HTTP/1.1 500 Internal Server Error
  < X-Powered-By: Express
  < Content-Type: application/json
   < Connection: keep-alive
  < Transfer-Encoding: chunked
  < 
  * Connection #0 to host spamcheck.postmarkapp.com left intact
  * Closing connection #0
  {"error":{}}

Wonder if you've got any pointers on how to do this at the shell...

I got it from here: http://spamcheck.postmarkapp.com/doc