About 2.5 gCO2e for a 1 MB email, according to ADEME's reference value (2.46 g), which includes the manufacture of the devices. ADEME's Impact CO2 simulator shows 0.11 g for an email without an attachment written in 3 minutes on a phone, without the manufacture of the phone; with it, that email comes to 1.6 g by our calculation. The 4 g and 19 g figures that are still quoted date from 2010 and 2011.
Read the source article: Carbon footprint of an email: about 2.5 gCO2e for 1 MB