  Internet services contribute a large fraction of worldwide carbon emission
nowadays, in a context of increasing number of companies tending to provide and
more and more developers use Internet services. Noticeably, a trend is those
service providers are trying to reduce their carbon emissions by utilizing
on-site or off-site renewable energy in their datacenters in order to attract
more customers. With such efforts have been paid, there are still some users
who are aggressively calling for even cleaner Internet services. For example,
over 500,000 Facebook users petitioned the social networking site to use
renewable energy to power its datacenter. However, it seems impossible for such
demand to be satisfied merely from the inside of those production datacenters,
considering the transition cost and stability. Outside the existing Internet
services, on the other hand, may easily set up a proxy service to attract those
renewable-energy-sensitive users, by 1) using carbon neutral or even
over-offsetting cloud instances to bridge the end user and traditional Internet
services; and 2) estimating and offsetting the carbon emissions from the
traditional Internet services. In our paper, we proposed GreenMail, which is a
general IMAP proxy caching system that connects email users and traditional
email services. GreenMail runs on green web hosts to cache users' emails on
green cloud instances. Besides, it offsets the carbon emitted by traditional
backend email services. With GreenMail, users could set a carbon emission
constraint and use traditional email service without breaking any code
modification of user side and email server side.
