@matthewtoad43 In 2010, Mike Berners Lee calculated it was 4 g.
Here's some 2022 figures:
Spam email picked up by your filters 0.03 g
Short email sent and received on a phone 0.2 g
Short email sent and received on a laptop 0.3 g
Long email that takes 10 minutes to write and 3 minutes to read sent and received on a laptop17 g
https://carbonliteracy.com/the-carbon-cost-of-an-email/
I think an average email at 4 g sounds reasonable