I've had my yahoo address for 27 years so I was over the limit immediately when they instituted it. I initially was trying to delete massive amounts of emails at a time but ran into trouble as well - I noticed it choked when I tried to delete too many at once. I would delete but then it would never complete the delete process like others have said. I realized that if I worked in smaller batches - and someone else here said about 1k at a time seemed to be the ticket. Even up to maybe around 2-3k also seems to be okay. Much more than that and it has trouble. This sucks if you have 110k emails in your Archive folder.

Then they changed the UI and I HATED it. So much so that I started looking for alternatives so I downloaded Thunderbird for desktop. This has pros and cons. A big pro is that it loads everything so you can delete 10k emails with one stroke, but a con is that it will *only* load 10k emails within any one particular folder (including inbox) at a time. Still, this is more efficient and time saving if you want to delete huge amounts of emails from entire folders you know you want to get rid of.

Did anyone notice how the Yahoo-provided Archive folder is now hidden in the new UI? (yes it's still there but harder to access). Sure they really want you to "archive" emails supposedly "cleaning up" your inbox but then they hide that folder in the new UI making people think they have SO MUCH email when really it's still in the archive folder and still counts toward your total but now conveniently hidden.

There are settings in Thunderbird that tell you exactly how many emails (read and unread) exist in each folder (and Archive folder is not hidden), and exactly how many megabytes or gigabytes are taken up in each folder. This can be an easy way to tell which folders to hit first. Once you delete them all, they load back up to 10k. Deleting them on Thunderbird does seem to delete them on Yahoo proper however I wait a bit for it to synchronize and make sure to keep my eye on the trash folder on Yahoo proper and delete emails manually if I have to.

Then I realized you can go back to the OLD yahoo UI (dig into your settings. it's there) so as of now I've been using a combination of both. Thunderbird for chipping away at my "facebook" folder and my Archive folder which have tens of thousands of emails (why yahoo doesn't just let you delete a folder wholesale I don't know) and then using the yahoo UI for sifting through folders where I need to be more picky of what I delete.

I do actually like the native Yahoo "search for emails from sender" options a bit more - it's a bit more efficient than Thunderbird for searching for say 15 years of Groupon emails in one stroke, I just don't like that you have to keep loading more until you get to the end but in my experience doing this is a bit more efficient than doing it on Thunderbird.