Your comment displays a serious lack of of understanding of how Thunderbird actually manages mail. Scripts do not run. So it does not matter what malicious payload the mail contains. You can open it, dissect it, check it's source. It is harmless. This is not so in most Microsoft mail readers or web mail which will execute scripts. But it is so in Thunderbird. Similarly any malicious content in an attachment would remain inert as mime attachments are not decoded until you actually chose to open them. The only possible exception would be through displaying images and remote images are disabled b default.