Deleting large number of emails at one time

I believe this still works.

Mass Deletion of E-Mail in the ATT Webmail (How to erase them all at once instead of 50 at a time)

1. You must be using "Full-Featured" email, not Basic.
2. In the Search box at the top of the mail page, click the word "All".  Then click "Advanced search" at the bottom of the list.
3. The dialog allows you to define search criteria: All folders vs just Inbox, for example.  Note there is an "AND" between the items you define.  Therefore, if you enter your email address in both the "To:" and "From:" fields search will only present the emails you sent to yourself.  That means you may have to do this twice (first using To: you, then From: you) to delete everything. Note you can also define a date range, Subject, etc..
4. When the search details have been defined, click "Search mail".
5. The top row will show the number of items found.  You can click "Refine" to narrow the results.
6. Click the very top checkbox to select all the results.
7. This is a good time to stop and reflect: DO YOU REALLY WANT TO DO THIS?
8. Click Delete.

Even with the above suggestion ,I had to scroll through pages of results.  If you have not scrolled down, the 'Select all results' only truly selects the emails that have been loaded onto the page in your browser.  This is non-optimal.  

Here is what I finally did - For My inbox, which had the majority, I set up GMAIL to fetch all of my emails from the ATT (yahoo) account.  I made sure to uncheck the box to leave a copy on the server.  I told GMAIL to skip my GMAIL inbox and to tag all of the mails with something I would recognize.  I think was able to select all of the emails that had been downloaded and delete all with one click.   - Note here - If you do this - Make sure you turn off the sync in GMAIL when you are done, or all of your mails will continue to get downloaded to GMAIL when they arrive, basically skipping your yahoo inbox.

For my archive mail, I had to just scroll down 30 or 40 times until I had 4-5K emails loaded then 'Select All' as described above.  Then delete that block of 4-5K.  Then I would repeat the process until I had cleared all that were in Archive (and other folders).  Once that was done, I went to the trash and clicked 'Empty Trash'.

Note - Before I did any of this I used one of the most excellent features in the Yahoo mail system - The pictures and Attachments functions.  This let me save off important documents and photos before I commenced to cleaning everything up.

Now I have a completely fresh starting point.  No more 13 years worth of old emails bogging me down.  

What would be truly optimal would be a setting to delete any unopened email that was more the xxx months old.  If I have had an email in my inbox for more than 2 months and have not opened it , it was not important enough to keep.  Maybe one day :-)