The issue that you are seeing is being caused by the Apache Webserver module known as "mod_security". This module provides system administrators a method for creating rules to reject certain requests based on information in the request, such as the User-Agent (the string that identifies the web browser or service making the request).
It appears that you (or your provider) have a rule which is being overly aggressive in its matching and is catching the dlvr.it User-Agent.
Our User-Agent is:
If you are running your own servers, we would appreciate knowing which ruleset you installed on your servers that is causing the issue.
If you are operating in a hosted environment, we would appreciate knowing which host you use so that we can reach out to them to have the change applied, and find out which ruleset they are using.