Why can't Apple's Mail app delete multiple messages?

I didn't discover a 'fix.' As Mac Jim ID wrote; if Apple Mail is mirroring my Yahoo email account, Yahoo may not have had time to delete all the ones I've selected before Mail 'syncs up' with Yahoo. I'm guessing that Mail deletes the hundreds or so emails, then immediately checks Yahoo to 'mirror' it, and Yahoo hasn't yet deleted them, so Mail keeps showing them. What I did was to select a batch, but less than the big batches I originally selected, then hit the trashcan button. So far, that seems to work. Not ideal, but smaller batches maybe give Yahoo time to act. Again, just guessing. You may have to experiment to find an optimum size for your batch. And I think performance may fluctuate, like if Yahoo (or the server you're using) is having issues, it may be slower on some days than others, so sometimes the problem might recur. I hope that makes sense...

Thanks. The email provider is Yahoo. I suspect you are right. I tried multiple deletes today, and didn't select as many to delete in one go. I also waited a second longer before hitting the trash icon (don't know if that really helped or not): so far it seems the deleted messages are really deleted. I'm still experimenting with it to see if the problem in my situation is just selecting too many at once, where what you described about the email provider not yet deleting them comes into play. I appreciate your input.