Yesterday, I managed to configure HEP for mail2blog operations. While the user manual concisely lists what needs to be done, I thought adding some remarks to it would cause no harm. I installed MT in a /cgi-bin/ directory on my server, and have a number of blogs running there, all accessible across http://blogs.cocoondev.org/.
In my HEP configuration panel, I added a new message destination of type 'MetaWeblog API'. Here are my message destination parameters:
Name: OuterWebThoughtLog - this is a symbolic name, you can pick any name you want here.
User Name: stevenn - your MT user name
Password: your MT password
Blog ID Number: MT assigns a number to each weblog - the easiest way to obtain that number is going into the MT Main Menu (the one which appears after logging into http://host/cgi-bin/mt.cgi), and check the URL of the 'Manage Blog' link of the weblog of your choice: http://host/cgi-bin/mt.cgi?__mode=menu&blog_id=2 in my case. My Blog ID is 2 here.
URL: the XML-RPC connection point of your MT installation - most likely something like http://host/cgi-bin/mt-xmlrpc.cgi.
OK - that was one part of the setup, the other one requires you to configure your email client. Possible caveats are forgetting to punch holes in your firewall for the unusual ports HEP uses by default, and fighting with your mail client configuration to set your HEP server as your SMTP outgoing host for your mail2blog messages. With Mozilla, you can set up an SMTP server per mail account so that works out well. After having done all of this (which is well explained in the HEP User Manual), you are ready to send mail using that SMTP server. Just make sure your To: address is correct: OuterWebThoughtLog@hep (yes, that symbolic name, and it is case-sensitive apparently). Hope this helped!