Concerning external tools/apps, we do not have specific recommendations for a local email client for SMTP-IMAP/POP3 connection (nor do we endorse or take any responsibility in terms of their security/privacy).
However, to facilitate the use of your of Mailfence account in order to achieve end-to-end encryption on external tools/apps, here are some of the common third-party tools/apps that you can use based on your needs:
End-to-end encrypted email: OpenPGP supported
End-to-end encrypted email: OpenPGP plugins
- iPGmail for iOS
- Windows 10 Mail
All of the necessary connection settings can be found in your Mailfence account Settings -> Account -> My connection settings. Concerning the app/tool specific instructions of setting up your Mailfence account, please refer to the respective manual/user-guide of the respective app/tool you are using.