Questions
Why I can't find other Confluence users in the recipient field?
Mail Page Cloud can only access the email address of Confluence users
who have either set the visibility of their mail address to "anyone" in their Atlassian profile under "Profiles and visibility"
or who are registered as managed accounts in the Confluence instance (recommended).
Using managed accounts is recommended by Atlassian anyway for security reasons and is a reasonable configuration for your instance regardless of Mail Page.
Here is more information from Atlassian on how to set up managed accounts as Confluence administrator:
https://support.atlassian.com/user-management/docs/what-are-managed-accounts/
https://support.atlassian.com/user-management/docs/verify-a-domain-to-manage-accounts/
After you verify your domain for managed accounts and claim the accounts, the visibility of the email address of these accounts is then automatically set to the entire organisation so that Mail Page can access it.
The "Profiles and visibility" of a managed user account should look like this:
You should then be able to find and select all other Confluence users in the organisation in the recipient field of Mail Page Cloud.