Are you using the native Mail App?
Many mailbox folders have a Select All button at top-left of the Mail sidebar - and other action buttons at the bottom of the sidebar such as Mark, Move & Bin. For the few folders that don’t have Select All, you can still easily select and delete multiple emails.
1) Select a mailbox folder (e.g., inbox, archive, bin, sent etc.) from the Mail sidebar.
2) Having selected the folder, use the Search field or any filter options that you wish to use (e.g., Unread/Marked) to refine the list of mail messages.
3) Tap the blue Edit button in the sidebar.
4) To select multiple messages, you can either tap (select) each message individually - or simply swipe (multiple select) your finger down the left edge of the sidebar to rapidly select multiple messages from the list - sliding your finger beyond the bottom of the list to select more messages.
5) When you’re ready, select the desired action (e.g., Bin) from the available options.