I'm trying to copy a large amount of emails over to another account. Since Gmail is unique in structuring it's emails, I'm unsure if all the emails I've copied are successful. I'm trying to figure out if all of my emails have been copied, but considering the email queue has stopped on occasion and I noticed a few emails missing, I'm not quite sure if I copied the full spectrum over. I notice when I go to copy an email again, Thunderbird says, "Copy to [Inbox] Again" option, or something similar. So, Thunderbird seems to know it's copied it. In that case, Is there a way to filter out what emails have already been copied so I can see which emails have never been copied?