That’s strange! It works fine for us…
Have you/hosting company made any changes to your server? The plugin sets the mail type to html for instant send and queued email so it should work.
Have a look at the message source in your email program and see what is in the header section – it should have something like
Content-Type: multipart/alternative; or Content-Type:text/html in it.
If not something has happened at the server level or Mailbank (no idea what that is by the way!)