Tip for the safest mail client?

There is no such thing as a safe email client, or an unsafe one for that matter. What's important is your provider and even then email is inherently unsafe unless you use something like PGP.

Well, since you dont define your threat model ... i'll just leave this here https://www.privacyguides.org/en/email-clients/ Hope it helps.

Start looking at safest e-mail providers before looking at safe e-mail clients. But yes, one of the safest.

protonmail.com account with https://github.com/vladimiry/ElectronMail client