Studies on the subject (very few actually, if you have intel on that matter, let me know) have already been conducted and reveal that a simple email with an attachment of 1MB produces around 15 grams of CO2[1]. Obviously, this figure increases with the size of the email. This is the case, for example, when the email includes large attachments or if the email is sent to several recipients.
With the use of the IMAP protocol, one email sent has at least 6 permanent copies (from the sent item in the sender email client to the inbox of the recipient, through sender and recipients email server which hopefully have long term archiving).
A solution like firefox send with automatic shredding of the file after an expiration period to replace email attachment would drastically reduce the consequences of email usage on greenhouse gas emissions. It would also resolve other issues related to sending files by email, but that would make this post waaaayyy to long :-)
[1] http://www.helixee.me/limpact-ecologique-des-e-mails/ (in French)
I run my own infra and on my end I see 1 permanent server side copy and 1 temporary local copy.
2. local copy in the send item once sent
3. a copy on the sender email server
4. a copy on the receiver email server
5. a copy on the inbox of the receiver
6. add one if the file is saved as a local copy on the sender computer
And this does not take into consideration backups (like local backups and email server backups). Also, the number increases obviously by the number of recipients (points 4, 5 and 6).
(edited for layout)
It's 4 total if both sides cache the file indefinitely and 2 otherwise.
edit:
Okay, I see, you are talking about physical files.
Sure then there might be up to 6 copies of the file.