This may not answer your question on how to find users for your tool, but may be a suggestion to change "target audience" of your "productivity tool" from email recipients who get these sporadically to email senders
whose job it is to send them (i.e: productivity tool for someone whose job it is to do a task and who want to do their job better and who could spend money on your tool as and write it down as tooling cost as opposed to individuals rarely receiving emails who'll see it as a nice to have and pay for it from their own pocket: how valuable is it, really).
There's an asymmetry here: do people receive more emails from recruiters than do recruiters receive from candidates?
I suppose you could change your product to serve recruiters and solve the problems they have. Maybe your product will help them so much that people will not feel a need to create a tool to deal with emails from recruiters, thereby eliminating that problem as a bonus.
