The US Postal Service is studying the environmental impact of mail delivery. But they claim that advertising mail reduces harmful emissions, by informing consumers and so reducing shopping trips.
By my own back of the (recycled) envelope calculations, an airmail letter from from Canberra to Brisbane produces about 136 g of CO2 equivalent and this is one hundred times as much as email.
Here is the calculation:
CO2 from Paper Mail
A sheet of A4 paper weighs about 5 g.
An envelope and stamp will weight about 7 g.
This gives a total of 12 g for a letter.
For a flight of around 2500km, .1260 kg per km of CO2 is produced to transport a passenger.
The standard weight for a passenger is 77 kg.
So that works out to about 1.64 g of CO2 per km per kg of cargo, or 0.14 g per letter per km.
A letter which went 1,000 km (about the distance from Canberra to Brisbane) would produce about 136 g of CO2 equivalent.
CO2 From Email
My estimate is that a 20 kbyte e-mail message (one A4 page equivalent) produces one gram of CO2 per year <http://www.tomw.net.au/blog/2007/10/one-gram-per-message-program.html>.
So email would be much better, as long as you did not keep the message online too long.
However, if the letter was only being transported a few tens of km within the same city by road, then the CO2 emissions for the paper letter would drop to under one gram. This would then might be more than an email message kept online a long time.
Combined electronic and paper mail delivery
Obviously it is possible to reduce the impact of long distance paper mail by transporting it most of the way electronically and printing it near its destination. About twenty years ago I helped interface a system at the Department of Education to Australia Posts' system to do this. Setting it up was complex, but it worked reasonably well. This should now be easy to do with standardized Internet based protocols.
Australia Post have a service called eLetter, which seems to be for printing and delivery of mail. Unfortunately Australia Post seems have a very poor quality web site, making it difficult to find out about the service.
Also they seem to be concentrating on helping send more junk mail, with services such as Easy Post.
Large mail users, such as the federal government could send correspondence to the nearest capital city electronically for local delivery. Apart from saving greenhouse gases, this would save money. Setting up a system for the whole of the Australia Government would be no harder than the system I helped build for one agency twenty years ago.