Sample code to get Thread-Topic header (email conversation topic) from inbound email.

Get Email Thread-Topic.md

Thread-Topic is a header of email message, it contains the original subject of the conversation

var headers = email.headers + "";
var topic_regex = /Thread-Topic:(.*)/i;
var topic_match = topic_regex.exec(headers);
var topic = (topic_match && topic_match[1]) ? topic_match[1] : email.subject;