Yes, Arca PE’s encryption is compatible with all other software using PGP/MIME (and PGP/Inline) standard.
ProtonMail and other “secure email” services are examples of centralized email solutions: if an attacker breaches their servers, they gain access to both the emails and the private keys. In contrast, a well-designed email client is decentralized, utilizing any email service while ensuring the private key remains solely on the user’s device without their permission.
Even if an attacker breaches these servers, they cannot read the emails, as they lack access to the private keys, which are securely stored on the user’s own devices (computers, phones, etc.).