Updated: Apr 10
In general, we don't spend too much time thinking of the energy it takes to send an email. The computer (smartphone, tablet, whatever...) is switched on anyway and the battery indicator doesn't change one bit if we send an email.
One email, even with an attachment and a fancy video background doesn't make a difference.
Taking a global view, things look a little different.
1. The impact of internet on the environment
In an article that is already more than nine years old, The Guardian states that “around 62 trillion spam messages are sent every year, requiring the use of 33 billion kilowatt hours (KWh) of electricity and causing around 20 million tonnes of CO2 equivalents (CO2e) per year.”
The situation has not improved.
According to Statista,  in 2020 there will be more than 300 billion emails sent. EVERY DAY!
(a statistic that was drawn up before the idea of confinement due to a pandemic even arose!)
That is obviously an estimate and as such subject to some uncertainty, however, the order of magnitude is important.
The energy consumption of the Internet is enormous, and it’s growing rapidly.
(A) The impact of sending and receiving e-mails.
Take a second to think about it.
When you are typing an e mail, your computer is using electricity.
When you press send it goes through the network and it needs electricity to run the network.
And at the end, it will be stored on the cloud somewhere, and those data centres use a lot of electricity.
But we don’t think about it.
Sure, from time to time, we need to recharge the phone but we need to do that anyway, whether we send an email or not, and if we use a desktop computer that's plugged to the main power supply we don't notice a spike in our electricity bill when we send an email.
But the carbon footprint of internet is huge and growing.
Mike Berners-Lee  estimates that a typical year of incoming mail adds 136 kg of emissions to a person’s carbon footprint, or the equivalent of driving 320 km using an average car.
On a larger scale, he says that the world's data centres in 2010 accounted for 130 million tonnes of CO2e, or a quarter of a per cent of the world’s total.
Berners-Lee projected that the world’s data centres will produce 250-340 million tonnes CO2e by 2020. 
Both this article and the one by The Guardian (vide supra) deal exclusively with the electricity required to use the electronic infrastructure, not to build it.
And that's just emails.
(B) The impact of streaming videos
According to Netflix in 2014, the average customer had a carbon footprint of 300 g per year. Considering only the energy necessary to deliver the service, not the power consumed by the device.
Netflix has since made its service carbon neutral, including the power it uses through Amazon Web Services and its own Open Connect program. 
(C) The impact of streaming music
According to Brian Palmer, streaming an album 27 times used up the same amount of energy as producing and shipping a CD. 
We may feel that we are consuming less and better thanks to the internet.
But digital technologies account for more carbon emissions than the aerospace industry. 
Transmitting and viewing online video accounts for a large portion for this, generating nearly 1% of global emissions.
2. So, is the internet good for the planet?
On the whole, the internet saves a lot of energy.
The energy needed for a single internet search or email is small. Consider having to send all your emails as traditional letters or postcards.
Transporting all that paper and cardboard around would require enormous amounts of fuel.
Producing all that paper would require enormous resources as well.
It is not unreasonable to assume that we would reduce our mailing activity by orders of magnitude if we had to do it the old fashioned way again.
The problem is that because the internet saves so much effort and resources it is used in irresponsible ways.
According to the ITU, 53.6% of the global population, now use the internet. 
So the energy and associated gases emitted with each online activity add up.
Mike Hazas, researcher at Lancaster University, stated that the carbon footprint of our gadgets, the internet and the systems supporting them is similar to the amount produced by the airline industry globally; and is predicted to double by 2025! 
3. What can we do?
It is unlikely that we stop sending e-mails or watch a video on on Youtube tomorrow, so what can we do?
Reduce the amount of back and forth emails, not to mention spam.
Unsubscribe from email newsletters you don't read anymore.
These may sound like small suggestions not amounting to much, but if one email doesn't make a difference on a personal level then saving one email personal level still adds up globally.
As a large UK-based supermarket chain always says: Every little helps!
We could save millions, maybe even billions, of emails from taking up valuable space in the data banks of the world, sucking down countless kilowatts of energy, further aiding in the deforestation of the Amazon jungles, causing the climate to further warm bringing ever closer to the brink of destruction! 
The aforementioned article in The Guardian ends in (not quite seriously) suggesting a tax of 1 penny per email, conceding immediately that this obviously not going to happen for various reasons, most importantly, because there is no global tax authority. But, a nice thought nonetheless.
In all cases, the question of how harmful any given activity’s energy use may depend in part on the company you’re using. Several companies, including Apple, Google and Netflix, have all committed to using clean sources of energy for their data centres – and, in many cases, are increasing the percentage of clean energy they’re using for their services each year.
The clue will be education and awareness to help people be more conscious costumer.
As we are dealing in communicating science this is our business.
Awareness of the problem would come from two angles,
the problem would have to be dragged through the mass media more often and
the individual user would have to get a feel for how much energy they are using, and that is not just how often they have to recharge their phones but also and much more importantly an idea of the follow-on energy consumption their web-activities entail.
Which brings us neatly to this old chestnut:
Act only in such ways which you can wish to become universal law.
An oldie but definitely a goodie.
And education is the key to the ability to make such decisions.
 https://www.theguardian.com/environment/green-living-blog/2010/oct/21/carbon-footprint-email [Last accessed on April 6th, 2020]
 https://www.statista.com/statistics/456500/daily-number-of-e-mails-worldwide/ [Last accessed on April 6th, 2020]
 There is no planet B : a handbook for the make or break years by Mike Berners-Lee
 https://www.independent.co.uk/life-style/gadgets-and-tech/how-bad-is-email-for-the-environment-a7555161.html [Last accessed on April 6th, 2020]
 https://media.netflix.com/en/company-blog/a-renewable-energy-update-from-us [Last accessed on April 6th, 2020]
 https://www.washingtonpost.com/national/health-science/depending-on-how-you-listen-cds-may-be-the-environmentally-sensitive-choice/2013/05/24/9a6ff63c-c15e-11e2-bfdb-3886a561c1ff_story.html [Last accessed on April 6th, 2020]
 https://theshiftproject.org/wp-content/uploads/2019/03/Lean-ICT-Report_The-Shift-Project_2019.pdf [Last accessed on April 6th, 2020]
 https://www.itu.int/en/ITU-D/Statistics/Pages/stat/default.aspx [Last accessed on April 6th, 2020]
 https://www.bbc.com/future/article/20200218-climate-change-how-to-cut-your-carbon-emissions-when-flying [Last accessed on April 6th, 2020]