How to re-set the date from which Thunderbird downloads POP email ?

My understanding is that there is nothing at the Yahoo end which signifies whether a message has been downloaded or not.  The choice of whether the message is downloaded or not is down to the email client (Thunderbird in your case) which stores a list of the Message Id's corresponding to each of the messages to be downloaded.  It looks at each message that comes down the pipe and compares its' list for each one, then marks it for download or not.

I suspect that when Tbird was installed it was not done as un upgrade, but as a new install, or a new Mailbox was defined rather than using the existing one, which meant that the list of downloaded messages was no longer available for inspection by the new version.

Your best bet would be to login to the Web interface of AT&T/Yahoo and delete the messages from there, but then the only copy of them will be in your old mailbox.  

One snag with leaving messages on the server is that one day your mailbox will become full and people will not be able to email you - you may not get a message from AT&T/Yahoo saying that your mailbox is full, but people emailing you will get a bounce-back saying that your mailbox quota has been exceeded.

in my understanding this is a server side issue.

See this screenshot from gmail:



28.12.png

Also yahoo mail now supports:

Now that  Yahoo! Mail features unlimited storage (wow!), the easiest option for archiving (i.e., saving) your email is simply to keep it here.

So space is not an issue here.

Well, that is the certainly the way to go if AT&T/Yahoo has equivalent options on its control panel.  

Hmm I wonder how big unlimited is?  Big enough in this instance, I'm sure, but are there no asterisks knocking around that word?  (lol)

Update,
my fiddling around has resulted in TB showing a willingness to download messages as they come in today, but so far I have a huge gap in my inbox from December 8th until today.
Therefore I am still looking for a solution.

Thank you moorhouselondon & Tolomir,
I have always assumed that the Read vs. Unread status in the web-based email is the flag that tells TB what to download.  But I am intrigued by the idea that there is some comparison of message headers.

I cannot find any place at Yahoo to specify a POP-related date on the Yahoo side.

I believe that my colleagues have huge mail stores at Yahoo without any problem at all, and I have not worried about it at all since they announce the unlimited policy.

Still wondering how to get those emails over from Yahoo to TB...

hmm, maybe you can set the date in windows to 9, 10... 27th of december then start thunderbird for each day and download just the new messages. this  might result an all-email-collection.

please set the date in windows to 9th of December and configure thunderbird just to download new messages.

Update #2,
Well, I have been looking around the web, and at least now I have a plan.
My plan is based upon the notion that the UNread flag on the Yahoo side may very well be ignored by TB.
TB seems to use a local file called popstate.dat which is in a local directory that is part of the TB "profile".  
Here is an excerpt from the beginning of the file

I think that the letter k means to keep the message on the server
and the rest of the line may be a way to uniquely identify each and every message.

At the moment I have two popstate.dat files.  One is in a directory called
 ...\Mail\pop.att.yahoo-1.com   (2KB)
and the other is in
 ...\Mail\pop.att.yahoo.com   (679KB) and has.. well well ...  16,854 entries

So, I plan to close TB and go over to web-based email at Yahoo.
I will move all the messages, except for a few,
 out of the INBOX and into some other folder.
(This will be tedious, because Yahoo will only show a max of 200 messages on screen at one time.  Then one can select all of those 200 and move them to a folder.  Repeat until the inbox is nearly empty.)

Then I will rename the popstate.dat files (thus temporarily making them invisible to TB, right?).

OK, then I will ask TB to Get New Messages, and see if the few messages from the inbox download correctly from Yahoo to TB.
I assume that popstate.dat would be created anew.

I'll report back later.

oops sorry I should give 25 points to moorhouselondon, but I don't know how to do that

Good piece of research which indeed will be helpful in future.  If you do wish to allocate points differently post a Question in Community Support.

Accept your answer as solution also any other comment.
Then divide points to the answers.
I think your points will be reduced to 0 while the others will be kept.