How can I select all [emails] after a search is returned yielding 100s of results?

Hello I think that you have two options depending on the device:

• On iPhone once you've done your search you can tap on 'Edit' and keeping one finger on the checkboxes you start to slide down and with the other you scroll till the end. Maybe you'll have to do it multiple times if you have many emails but is pretty fast.
• Otherwise if you have access to a mac you can search and tap on `command + A` to select all emails in the results.

On the phone you can do the search, click edit and then slide your finger up or down the list where the circles are to make the check marks appear…slide the page up (or down) and continue to repeat the process until all that you want are selected. Click trash.

Or log into the email account on a computer which may make the process faster. Depending on the email provider you can sometimes search for the ones you want and then select Delete All or Select All then delete