I think 4 grams of CO2 mostly means the delivery of the E-Mail. Also I think cleaning out your mailbox, especially when you're using Gmail, means it's still on their servers and backups 10-fold, probably.
An E-Mail stored on a storage media does only secondarily, maybe, cause more CO2 due to more storage being bought to store more of them. With out current density though
Though this article is maybe also a good point of data: medium.com/stanford-magazine/carbo...
Nontheless, searching for and removing emails causes even more of a carbon footprint in a web browser expensively rendering web pages, the web server chugging for the data, database servers, etc... Which might tip the scales a bit. A Google Search apparently has 0.2g of carbon footprint, but given many datacenters use green energy nowadays (does your home?) that might be neutral.
Overanalysis of a topic, good way to spend my time.
Anyway; how about reducing emails you get at all? Sign out from some newsletters. Use a laptop more instead of a tower (if you own both, buying one or the other is a crazy footprint), they're very energy efficient.
Or check out the good old outside instead of watching media, also helps yourself.
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