I noticed that I had 51.6mbytes of space used up in hundreds of messages. But 35% of this was from just 28 messages which had large attachments. These were messages I did not really want to keep (and would have preferred not to get in the first place). The rest of the messages were very small. Ways to prevent such messages being received might provide large benefits for the capacity of systems.
As an example, if email attachments attachments were kept on the sender's server and only sent when requested (and in most cases never requested), that would reduce email traffic. Perhaps a useful email utility would check to see if a document a person was attaching to email was already available publicly on the web and offer to insert a link instead.