I read through the questions like this, some identical, that have already been answered. I did the steps, selecting subscribe on the sent folder on the computer with missing sent mail, clicked download and I did not get the missing sent message that was sent on another computer yesterday. I do have all my incoming mail for the account. This is a gmail account, windows 10 and 11.

So I guess I have no idea what's wrong. I think the sent message should be on the gmail server since it went through there. I just looked, it is. And on the gmail webmail it is marked INBOX, a gray square. I looked at the incoming mail in inbox that I had replied to and there it is. This is on the gmail webmail. But in thunderbird, the reply is not appended. And it's not in sent on a synced system. Something Google decided to do with their webmail that is not supported by Thunderbird?