I use eM Client to send HTML E-Mails to our Mailing Lists:

- Open a POP3 Account;

 

- Create a new Message, Right-Click the Message Body, select Edit Source, past your HTML Page:

- Send to your Mailing List.

Works for me but might depend on your actual HTML source.

PS One can actually create Templates for re-sending HTML E-Mails when needed :).

If you are sending an HTML formatted email via webmail to a mailing list you'll need to click on the Code View button in the editor window first, then add your HTML source here. Once you've tailored it the way you want it, click the Code View button again and you should see the rendered version of it in your editor. The sent copy should also come through as HTML this way!