what does this message number look like.... Thunderbird does not use message numbers, so I am hoping it looks like something else that might be useful.

(message 6706)

nope, Thunderbird messages in the file use a byte offset from the beginning of the file to identify their start, numbers much larger than that. But really what is the issue?

I need to get access to the message to quarantine it.

Why not just empty the trash and compact the folders? Then that message will be gone.

On a serious level, does having that cold virus in a glass phial in a special case in a locked box worry you. The same can be said of any virus anti virus programs detect in your trash.

It is just a text document in an isolated location that you have already dealt with. If you were going to open it you would have already, and probably have. Even if you take no action, the virus in that message will not be causing an issue. It will sit in it's locked box until as tinitarry says you "empty the trash and compact the folders" it is certainly not something to waste your time on and is about as cl;ose to a false positive as you can get.

Thanks Matt