I remember reading about the alleged reason eBay acquired Skype.
Skype is a peer to peer network. The guys behind it were the guys behind Kazaa, an earlier peer to peer network.
In Skype's case, the network is used to send audio and video.
In Kazaa's case the network was used to send "files".
If I recall correctly the article said eBay planned to use the Skype network to allow buyers and sellers to communicate with each other, directly.
It's also interesting you mentioned SMTP. I do not think I have ever seen anyone mention it online but SMTP works well over a peer to peer network.
Peer to peer SMTP obviates the need for intermediary SMTP servers and third party "email providers". With peer to peer networking there is no necessity for users' data to be stored and forwarded by third parties.
Not to suggest anyone would be interested in that.