The "Free Download Manager" (http://www.freedownloadmanager.org/) is a tool designed to make multiple concurrent connections to a web server and hence bypass rate limiting controls by downloading parts of a file separately. From the site "It splits files into several sections and then downloads them simultaneously allowing you to increase your download speed up to 600%.".
This would be abusive enough by itself, using more than your "fair share" of server resources. We don't rate limit individual connections, but there is a limit to how many connections each of our servers can provide at once.
The problem is that our servers don't support downloading separate fragments of an attachment to an email, because we fetch then entire email part from the IMAP server each time, decode it and start streaming it to your browser. FDM would make us fetch multiple copies...
UPDATE: I have been able to restrict the block to just the FDM manager itself rather than any broswer with it embedded. This should make regular FastMail use possible, so long as you don't engage FDM to do your downloads -- Bron
even that's not the problem. The problem is that if there's any error, FDM seems to just keep issuing hundreds of requests, which floods the IMAP server with requests and fills up the disk on our machines with decoded copies of the email. Sure, we could probably make our software safe against this condition, but doing that and testing it would take time. In the meanwhile, we must block the clients that are performing the equivalent of a denial of service attack on us.