Asssuming IMAP, my approach would be to verify that the synchronization setting was for all messsages regardless of age and then

1. click file>offlline>download
2. when done, click help>troubleshootinginformation
3. scroll to 'profile folder' and click 'open folder'
4. exit thunderbird
5. click the imapmail folder and then the IMAP account folder
6. there, locate the inbox, inbox.msf, sent, and sent.msf files and copy the inbox and sent folders to wherever you wish.

Hi David,

Thanks for the prompt reply. Since yahoo had drastically reduced the mail box size, I have to resolve this issue soon. Before I could read your reply, I segregated the inbox emails to different folders so that any technical issue in downloading old emails with heavy attachments is ruled out. I have also moved the emails in sent mail folder to different sub folders. However, there are approximately 3200 emails still left in the sent folder. Planning to install thunderbird in another laptop with 2 tb storage and latest processor. Will these steps help in completing the download quickly?

Regards, Mani

There are a load if issues here. and you and not going to like what your hear at all.

Yahoo apparent have a hard limit of 10,00 email per IMAP folder. They just do not synchronise them after that. They do offer another server, that is said to offer only one way downloads, but documentation of that is sparse to say the least.

Yahoo also have a limit on the daily bandwidth you can use for IMAP or POP. It is a couple og gigabytes and most of those desperate to get their mail off Yahoo have significantly more mail than that to get. SO a full synchronization might take weeks. Depending on how much data is involved.

Yahoo observer "security through obscurity" and as such do not document limits they impose on their systems like the number of concurrent connections but anecdotal evidence is that 'Yahoo impose a limit of five connections this is fine for most IMAP uses, but falls over rather quickly if you have apple products (the have no governor facility in their settings. Each device will consume all available connections or if you have multiple devices accessing the account like say a phone, tablet and Thunderbird. You run out of available connections quite quickly even if each device is quite moderate with its demands. SO good practice is to have nothing else accessing that mail account while you try and download it.

IMAP downloads are not stable archival downloads, hence Davids advice to move the data as soon as it is downloaded. IMAP use at best a type of cache of what the server holds. If mail disappear from the server (are removed or deleted) IMAP says they will also disappear from the local storage as it is synchronized to the server

Note hat Thunderbird will download message headers first, then the message bodies including attachments. This makes a list appear in Thunderbird of messages in a folder relatively quickly, but at the time the list appears the downloads are rarely even close to complete with a heavy lifting still continuing in the background. I suggest you have a look at the activity manager (double clicking the status bar at the bottom of the Thunderbird window will show it) It will show the status of most of your mail fetch and indexing processes. It will also give information on connection failures and dropped connections. Just be aware you may have to drag the dialog quite wide to see the full text of each item. It does not wrap messages