This topic has been on my mind a lot too.
To this end, we emailed about 400 users last week who met certain criteria (mainly English locale & iNat.org site affiliation) and fell into two main categories:
-2000+ observations and fewer than 200 identifications for others
-500-2000 observations and 0 identifications for others
Here’s an example of the personalized email they received (this one populates my user data from last week).
Click rates were pretty low (~10%) and I don’t think this was particularly successful. I think we’ll probably have better success in developing appropriate prompts in the website or app to guide people towards making identifications rather than heavily investing in an email-based strategy, but I wanted to give it shot.