Just a word of warning about trying to do mass deletes via an e-mail client: They will often bog down and go into "Not Responding" mode under Windows when there are thousands upon thousands of messages to deal with. I have recently been assisting several clients with e-mail client changes, both to Outlook and to Thunderbird, and both "get cranky" when the scope of messages selected is really, really large.
If you want to do a big time purge I strongly suggest using Gmail's webmail interface and the instructions I posted earlier. Even using that method you can sometimes see Gmail take a bit of time before coming back after the Delete is triggered, but it doesn't result in a hung program.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
The terrible state of public education has paid huge dividends in ignorance. Huge. We now have a country that can be told blatant lies — easily checkable, blatant lies — and I’m not talking about the covert workings of the CIA. When we have a terrorist attack, on September 11, 2001 with 19 men — 15 of them are Saudis — and five minutes later the whole country thinks they’re from Iraq — how can you have faith in the public? This is an easily checkable fact. The whole country is like the O.J. Simpson jurors.
~ Fran Lebowitz in Ruminator Magazine interview with Susannah McNeely (Aug/Sept 2005)