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 email address to "anyone" in their 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 on how to set up managed accounts from Atlassian:
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 shared with the entire organisation so that Mail Page can access it.
The "Profiles and visibility" of a managed user account should look like this:
