Magic Earth is not an open source app. They haven’t released any source code. Mentioning it as the OP title is about open source maps app.
https://www.magicearth.com/faq-en/
Will Magic Earth be Open Source?
No; since it is also used commercially (we have a paid Magic Earth SDK for business partners), we cannot make the code public.
That being said, I love using Magic Earth for driving. It works quite well as long as your area is up-to-date on OSM
If you want free, I’d recommend Protom Mail.
For paid, I really like Mailbox.org and Posteo. They work with IMAP/POP3, CardDAV, and CalDAV