I currently recommend avoiding using, and/or contributing to their products, until they fix their false advertising.
They are actually licensed under CC-BY-NC-SA (Non commercial, share-a-like), which is not an open source license (as commercial use allowed is a requirement for open source).
When confronted on Github, the issue was shortly closed, saying that the libraries used for the apps are open source, and that they have no plan for allowing commercial use for their apps.
I pointed out that open source requires commercial use, and I got this reply from their CEO:
"Commercial use is not a requirement for open source software. Study the MongoDB case."
Didn't know that MongoDB is open source. Because it's not. It's source-available, not open source.
Well, can't blame for users not checking this. Even It's FOSS News publication (https://news.itsfoss.com/skiff-mail-review/) got baited. Startups are using open source software's reputation, without contributing to the community (by using an proper license).
