Are you referring to the Allmail folder? If so, it is just a duplicate of messages in other folders and not generally needed. Maintaining it slows Thunderbird, increases disc space, and often causes confusion. BUT, if you want it, you can normally highlight the account, rightclick and select 'subscribe' and put a tick mark on the AllMail folder. And maybe I misunderstood your issue... Please share more info.

I feel, i could not explain my problem. As a beginer, I used thrunderbird with my gmail account details. Now, I did not pay attention and option "Delete Gmail's copy" in POP Downloading setting was marked. Now I do not see any email in GMAIL in browser as all are deleted but all old emails are available in thunderbird.

Can I get those old email from thunderbird to gmail server back ?

What you experienced is normal for POP accounts. Mail is downloaded to computer and deleted from server. Let us know what you were wanting to do, and maybe we can give advice on how to achieve that for future messages and that may help in explaining how to put old messages back on server.

@David, thanks for support. Is there any way that I get all deleted emails from server back to server ?

Yes, it can be done. You can set up an IMAP account alongside the POP account. Once done, you can copy messages from the POP inbox to the IMAP inbox, but that needs to be done a few at a time, as data can sometimes be lost when uploading if you attempt to copy a lot. One possibility to minimize the risk is to create a filter to do the copy. Once complete, you can then delete the IMAP account.

Thank you david. It works.