generating public/private credentials for MS Outlook email encryption

An app running on a CentOS 7 web server needs to receive encrypted emails sent from users of MS Outlook.

Can someone please provide explicit instructions as to how I can generate or procure all of the credentials that those Outlook users will need in order to send encrypted email to my app? This would of course include how I would generate or procure all the credentials that my app would need to decrypt the inbound emails received from users of MS Outlook.

It would help to know definitively what credentials need to be created as well as how to create them. Like am I using S/MIME, and what else? The Outlook users are in other organizations, so I cannot force them to install new software or add-ons to outlook, etc.

The fact that this is a Linux server with an app that does all processing of email with algorithms means that I cannot install Outlook on the server to handle email. Do I have to set up a remote IMAP connection and use Outlook to generate public and private encryption files, and then port the private keys/files to the web server for use by the Java app on linux, and distribute the public keys via email? Or is there some other way of generating the tools that is more Linux friendly and does not require remote IMAP with Outlook?