[quote]If you viewed the source, you have in fact opened the message/quote

Really? I've done that for years thinking it was a safe way to see if an email was safe to open or not. I've always thought of it as the very best reason to use Thunderbird.

IS there a way to see if an email is safe or not?

I received one supposedly from my own son but both the sender and the subject names were his name, so I checked it out via View>Message Source, thought it looked suspicious, called him and he had not sent it.

Someone had gotten into his email addresses somehow. I recognized names of other relatives and called them to warn them not to open it but their virus protection programs had eliminated it.

I followed your instructions and managed to send this email to spoof at paypal, but by doing so I could read the message, so apparently going through that opened it too.

I've just run a virus scan on my machine. Supposedly OK.

Now I suddenly feel like I should never open any email, ever again. This is disheartening.

Opening email is not in itself especially dangerous. Well, this is true in Thunderbird; in other mail clients, which run scripts, you are more exposed and vulnerable to bad content.

Clicking on stuff you find inside email messages is dangerous. Don't do that and it's hard to see how viewing a message can cause your computer any harm. I guess one advantage of viewing the source of a message is that the viewer doesn't render URLS etc as clickable links. But for that source viewer to work, the message has already been parsed and loaded into memory. I don't know how one could consider that not to be equivalent to "opening" it, whatever you think that means.

Viewing when offline, or with html disabled defeats the "phone home" webbugs that track your reading of the message, but these in themselves can't do anything to your computer.