I would like to have backup copies of all my Yahoo correspondence on my local laptop.

Since your Yahoo account is a POP account, all messages for this account have already been downloaded to your computer. This is inherent to POP. For a backup, backup the entire Thunderbird profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

The IMAP account is downloading ALL my received and sent messages into its Inbox folder.

I think this is a silly idea, since you download all your Yahoo messages twice. And IMAP is by no means a replacement for a proper backup.

There seem to be over 39,000+ messages in my Yahoo.com account!

I'd hope they are not all in your Inbox.

Local Folders is where I wish to create sub-folders to separate and store all the messages now being downloaded into the IMAP account's inbox.

I guess we've already discussed this at https://support.mozilla.org/en-US/questions/1079580 I still believe this is not possible, unless you manually move messages off of the IMAP server to your Local Folders account. But then again, you download everything twice.

during the downloading process, I received a message to the effect that the Inbox was full

That often indicates some sort of corruption. Try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

After moving messages off of the server to Local Folders don't forget to compact the IMAP folder. This add-on helps to know when compacting is needed. It shows the size of the local (IMAP) folder and the size on the server. When the local size is significantly larger than the size on the server compacting is recommended. https://nic-nac-project.org/~kaosmos/index-en.html#foldersize

Note, the add-on seems to have been abandoned, and the cert for the site has expired. It still works just fine for me though.

For christ1 --

After reading your post, I have the feeling that there is a bit of misunderstanding of what I have described as MY problem. First of all, allow me to further explain the situation that I am faced with. The problem of being ‘unable to move IMAP account Inbox messages to a Local Folder’ is not really MY problem, but rather the problem of a colleague who has been a loyal Yahoo.com user for many years. He has used Yahoo.com as his sole connection to the outside world and has collected well over 39,000 messages (I presume received and/or sent), which he has left either in his Yahoo ‘Inbox’ or that have been placed in the Yahoo ‘Sent’ folder by Yahoo. He has told me that he often deleted messages that are of no interest to him as well. His knowledge of “folders” is next to non-existent or even abysmal!

Furthermore, he is unfamiliar with forums such as ours or of any other type of Internet forum that offers assistance in solving problems. Secondly, he frequently contacts me via email or phone to provide solutions for dealing with his daily problematic interactions with Yahoo.com. Because of that, I had frequently proposed that he begin using an email client, specifically “Thunderbird”, simply because I have been using Tbird for many years with few or no problems of the type he has experienced with the Yahoo.com webmail.

My suggestion was eventually acceptable to him, so we downloaded the latest Tbird from the Internet and installed it on his laptop. I then created for him one POP3 account and clearly configured the Tbird’s account’s “Server Settings” to “Leave messages on server” “Until I delete them”. And that first day, 342 messages were moved from his Yahoo ‘Inbox’ to the Tbird ‘Inbox’. I then helped him to create some sub-folders under his POP ‘Local Folders’ and instructed him in the basic use of folders and filters within Tbird. He was amazed at how simple the process was!

After that first interaction of Tbird and his Yahoo account, he realized that the great bulk of his personal correspondence (both received and sent) was on the Yahoo server and that he really had no control over his correspondence whatsoever. He asked me if it would be possible to create a backup of ALL his received and sent emails that could later be put on to DVDs. I told him that I had recently had done something similar with my own emails on the Google Gmail server backup service called “Takeout”.

Several Internet searches brought to light that Yahoo had nothing like Google’s handy “Takeout”. And that is why an IMAP account was created in his Tbird. I explained to him the major differences between POP & IMAP accounts; POP grabs the mails, downloads them and deletes them from the server, while IMAP simply passes on copies of its emails keeping the originals on the Yahoo server. To top matters off, his Internet connection is not very reliable; it would frequently leave us stranded with broken download sessions between Yahoo and the Tbird application on his laptop.

During my recent visit with this particular colleague who lives in Bangkok, I offered to bring his problem to the attention of the forum and created a second account on his Tbird, the IMAP one. I began using IMAP nd it began to copy/download messages from Yahoo.com and deposit them on my colleague’s laptop in the folder “Inbox” under the IMAP account. As there are literally thousands of messages, the transfer is still only partially complete. Why? His Internet connection is from a Bangkok condo which provides Internet access only from 07:00 to 19:00 hours. And as I have already mentioned, his Internet connection is not very reliable so I have been cut off a number of times while transferring messages.

My own visit to Bangkok ended with IMAP still transferring messages, but the total messages has yet to be completed. I am no longer in Bangkok, but 862 kms away in northern Thailand and my colleague does not feel able to sit at the laptop for hours at a time watching for any interruptions in his Internet access. Consequently, as he wants to continue using his Yahoo.com account, I could not forbid him to do so until the time when I can visit him again to continue with the business of having IMAP complete the total transfer or copying of his messages to his laptop. So the whole process is now somewhat in limbo, it seems to me.

I explained in my original post that I had successfully created a “Recd via IMAP” sub-folder in Tbird’s ‘Local Folders’ and had been able to move, separately, through the normal cut & paste or drag & drop methods, four messages from the IMAP Inbox. The problem is now that I was able to move ONLY 4 MESSAGES.

Help is now needed to resolve that problem. I intend to print out the information or the 2 suggestions you have provided, i.e. https://support.mozilla.org/en-US/questions/1079580 and https://nic-nac-project.org/~kaosmos/index-en.html#foldersize for further off-line study and enlightenment and will report on their results once I have tried to use them.

Lets just short circuit this.

Install this add-on. https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Use it's menu (on the tools menu) to export the folders to the file system as EML files.

Backup complete.

The install and what to do might take a minute or 20, but the result is a repetition of something very simple, and should meet his needs.. As a bonus anything exported like that appears in Windows search.

I guess an unreliable Internet connection doesn't help when attempting to download thousands of messages. Do this only with a reliable connection. Also Yahoo isn't best known for a reliable email service.

When copying messages, don't drag and drop. Select multiple messages, then right-click - Copy To - choose the desired destination folder. Use copy instead of move. Verify messages have been copied successfully before deleting them in the source folder. With a large number of messages, don't do all at once. Copy them in batches instead.