It's certainly possible to send HTML messages to mailing list recipients via SmarterMail, and it's really pretty simple.
When sending via webmail, SmarterMail defaults to the "design view" when composing new messages. (Just like any other email client.) So, you can go ahead and use the HTML editor within SmarterMail to build your message, with formatting, inserted images, etc. and send it off to your mailing list recipients.
If you build the message outside of SmarterMail, and have the HTML code, if you want to send through webmail you'll want to switch your the compose window over to "Code view". Below is the button on the HTML editor to do this:
Once you switch to "Code view", you can paste the HTML code in the editor, then click it again to go back to HTML view and see what the code looks like. Once you've tweaked it to your liking, the message can be sent to your subscribers.
Realize the webmail client in SmarterMail works the same as any other email client. These, generally, default to a "design view" as well, so pasting raw HTML into Outlook or eM Client compose windows will just display the code when sent. In fact, external email clients basically force you to use their interfaces to build your emails. (There may be a way to use a "code view", but I couldn't find it after just a few minutes of searching.)
The nice thing about the web client in SmarterMail, however, is that it gives you the option to use that "Code view" button and paste your HTML into the editor, allowing you a bit more flexibility than traditional email clients as you can build the message in your editor of choice.