This is just silly. Assuming their estimate of 16433 t of CO2 per year[0] is correct, a person sending one e-mail less per day, throughout the year, would reduce their annual carbon footprint by:
16433 / 60e6 / 10 * 100% = 0.0027% [1]
See also MacKay's Without Hot Air take on a similar topic.[2]
Since OVO is an energy company, they definitely should know better. Charitably, they're being extraordinarily stupid. Uncharitably they're trying to push their "Carbon Capper" chrome extension which is probably spyware or trying to distract people from more serious issues.
[0] https://www.ovoenergy.com/ovo-newsroom/press-releases/2019/n...
[1] The UK has a population of about 60 million and the CO2 footprint is about 10 t per person.