[Karunamon]

An average email is roughly 75kb[1]. That same 75kb, plus TCP overhead, has to touch every single email server between you and the destination (wasn't able to find anything that suggested how many hops are average).

Some random googling specifies a number around 175 billion for number of spams sent per day.

That works out to an average of 12,223 terabytes per day - of just spam. Now multiply that by the number of hops that each message take. Assuming each message only has to touch one intermediary server between source and destination, that's still 3.6 petabytes.

[1] http://email.about.com/od/emailstatistics/f/What_is_the_Aver...

[Retric]

Spam tends to be a lot smaller (~6.4kb) as longer messages take more resources to send and are easier to detect. https://www.trustwave.com/support/labs/spam_statistics.asp

Also, Email does not need to hop from mail server to mail server all that much due to DNS. Granted legitimate mail might move around a fair bit, but as far as the public internet is concerned the vast majority of spam is sender -> possibly senders mail server -> spam detection software -> /dev/null.