Most secure email clients (applications installed in the system) for happy clickers are: Outlook, Thunderbird, PostBox, Mail for Windows 10 (Windows app), and Spike.
Anyway, Outlook is a part of MS Office which is the most vulnerable & exploited office suite, so it is probably not advisable for the home users.
The user can run executables directly from Spike, but they are downloaded with MOTW and checked by SmartScreen AppRep. Others do not allow running executables, the files are only downloaded to disk without execution. ThunderBird and PostBox download files with MOTW, so they are checked by SmartScreen AppRep when running from hard disk. Outlook and Mail for Windows 10, block executable attachments, so they cannot be even downloaded to disk.
Other popular email clients like Foxmail, Mailbird, Mailspring, Hiri, allow executing files directly from the application and the file is not checked by SmartScreen.