Pro FeaturesTo select every thread in the current list, click in the thread list and press Ctrl+A (Windows / Linux) or Cmd+A (Mac). You can then archive, move, label, or delete all of the selected threads at once using the toolbar.
To select a smaller set of threads, Shift-click to select a range, or Ctrl-click / Cmd-click to add individual threads to the selection. Hovering over a thread also reveals a checkbox you can use to build a selection with the mouse.
Mailspring loads long thread lists in pages as you scroll. "Select all" applies to the threads that have been loaded so far, so for very large folders you may need to scroll down before selecting to include older messages.