The carbon footprint, in carbon dioxide equivalent, of sending an email message, has been estimated at 0.3 gCO2e for a spam email, 4 gCO2e for a "proper email", and 50 gCO2e for an "email with long and tiresome attachment that you have to read". These figures have been repeated in the media many times, even recently in 2018, in 2017, etc., and implied to mean that it is climate-conscious to avoid sending too many emails. Indeed, with these figures, an average 121 emails per day would amount to 176 kgCO2e/year, which is certainly not negligible. My question is whether these figures are accurate (in 2020).
The figures seem to come from the 2010 book How Bad Are Bananas? The carbon footprint of everything by Mike Berners-Lee. I found them in the book, but no info about how they were computed, except a mention of research by McAfee without a specific source. I think it is probably this 2009 report, which does have the 0.3 gCO2e figure for a spam email, and a figure of 131 kgCO2/year in email-related emissions for the average business email user. But there is little information there about how the statistics were computed.
My reason to doubt this is that watching Netflix was more recently estimated (in 2020) to have a 28-47 gCO2e footprint for half an hour. Even taking the lower value, this would mean that a "proper email" would have the same emissions as 4 minutes of watching Netflix, and a spam would correspond to 20 seconds of Netflix. This is weird to me, as Netflix does seem to require much more bandwidth, computation power on the server and client, storage, etc. One explanation for why the email estimate seems high may be that the carbon footprint of network transfer has steadily decreased since 2009-2010: see this study Figure 3 for instance.
So I am wondering whether these estimations have been updated or refuted since 2009-2010. Looking online, I haven't been able to find anything that revisited or contradicted these statistics.