Prefer HTML to plain text will show HTML messages with their HTML formatting and layout. We strongly recommend to leave this option off, as security problems with HTML might show up. When this option is off, you can still read HTML messages, but only as plain text.
Load external references from the net will load external images, style sheets etc. from the internet when you look at an HTML message. We strongly recommend to leave this option off (although it has no effect if Prefer HTML to plain text is off already). By adding external references to their mails, people sending spam can detect that you have looked at their message. Note that this option has no effect on Java, Javascript and Plugins - these are disabled anyway and cannot be enabled at all.
Automatically send receive and read confirmations will automatically send a confirmation (in form of a new message) to the sender of a message if he requested that. If you leave this option off, no confirmation will be sent. We recommend to leave this option off. If you want to send confirmations to certain people you can do so by using the confirm delivery filter action.