Regarding using Yahoo server and version 148.0.1 Info here:

• https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo

I'm presuming you were already set up to use 'Authentication Method: Oauth2' for incoming and outgoing. So it should be a simple process to reset it.

Suggest you access the saved pasword and remove any line where 'username' = the yahoo email address. Then Exit Thunderbird, wait a few moments and then restart Thunderbird.

You should get a prompt from yahoo to enter password to confirm it's you and to allow thunderbird access.

Hi - thanks for your suggestions. I did check and specify Oauth2, and I didn't know about a 10,000 email limit. However, the problem certainly appeared only after a thunderbird update a few weeks ago. I am now using 149.0 on this computer, but we also use a laptop which hasn't updated since 140.9 and there is no problem with retaining emails there - it has the same accounts installed on it, all collecting new emails, and nothing is deleted unless it is done manually. In case it helped, I deleted a lot of older emails from my yahoo account inbox (several thousand), but it made no apparent difference. So the problem is definitely with changes in thunderbird software updates, but what, and how to solve it still defeats me.