imho: the future is a specialized compressor optimized for your specific format. ( https://openzl.org/ , ... )

This seems very cool. Was going to suggest submitting it, but I see there was a fairly popular thread 5 months ago for anyone interested: https://news.ycombinator.com/item?id=45492803

That is an interesting link.

Does gmail use a special codec for storing emails ?

The biggest savings for a service like GMail are going to be based around deduplication - e.g. if you can recognize that a newsletter went out to a thousand subscribers and store those all as deltas from a "canonical" copy - congratulations, that's >1000:1 compression, better than you could achieve with any general-purpose compression. Similarly, if you can recognize that an email is an Amazon shipping confirmation or a Facebook message notification or some other commonly repeated "form letter", you can achieve huge savings by factoring out all the common elements in them, like images or stylesheets.