that's really a misnomer. Once you view a message, it's been viewed, regardless of the name of the screen. you can set vertical view and see each message quickly and click again to open full screen. But if your concern is avoiding spammers, preview screens don't do it.

Agree with david It's impossible to not open and read anything. There has never been a 'Preview' - you or computer has to open file to see contents.

Thunderbird by default does not allow Remote Content unless you alter that settings - not advised. Opening an email is never a safety issue.

You should never click on anything in an email eg: a link until you have checked what that link really opens - hover over a link and it's real link will display in the bottom Status Bar.

You should never open any attachment unless you are 100% certain you know who sent it. Never open anything that sounds wrong or just because you are curious.

Those two things are relevant at all times.

Thanks for your replies. Think I've solved my problem by switching to Vertical View in Layout and then opening the Message Pane. That way I can see what the message is about and take avoiding action if needed.

You can toggle the message pane with F8. If you are used to Outlook and it's constrained interface the vertical view will appear familiar. Personally I use the wide view with the pane at the bottom occupying the full width of the window. It makes reading more relaxed I feel, rather that getting these very short lines of text in very long paragraphs. The vertical view is cluttered in my opinion even on a 27inch display and with the pane below I still see some 30 lines of new emails.

I do not understand your about taking avoiding actions if needed. IT makes no sense to me at all unless you are under some misconception that what you are doing is seeing some sort of preview. You have not previewed the content of the email, you have read the email. The same as if you had opened it in a new tab or a new window. If the email has a return receipt you will be prompted to send it just the same as if the email was read in it's own tab or windows. If the email has tracking images (Thunderbird does not enable remote images by default) that you load in the message pane, the sender will get the same tracking information as if you had read the message in other ways.