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)