Thanks @thrrgilag for shedding light on this!
Still doubt if Email is closed source by Jolla (never heard of this before), but it's another topic I think..
If it's open source they aren't exactly making it easy to locate. I looked at the github and gitlab locations and no projects or source for the email app. I've checked this thread https://together.jolla.com/question/41724/sailfish-os-source-code-location/ which states that their apps are closed source. I'd love to be proven wrong though.
Another question: I would like to try Dekko 2 on Desktop. Is it possible now? If not, does somebody trying to achieve that?
It's available as a snap. I have it running on my laptop in fact. Instructions are on the website. https://www.dekkoproject.org/download/