I would start debugging the issue by first sending the requests from Logstash to a service that just outputs the received request, for example RequestBin.
Something like:
output {
http {
http_method => "post"
url => "http://requestb.in/<created_id>"
format => "message"
content_type => "application/json"
message => "{\"source\":\"logstash\",\"from_address\":\"me@example.com\",\"subject\":\"Log Message\", \"content\":\"test\"}"
}
}
After you've made sure that the request Logstash is making is correct, take that request (preferably the exact data) and try to send it to Flowdock using curl
or some other means.
At this point you should be able to tell why the request fails in either end and notify the party accordingly (i.e. open a ticket to https://logstash.jira.com/secure/Dashboard.jspa or send an email to support@flowdock.com).