Unrecognized file formats are excluded from the search.
Certain types of files, such as Bitmap or MP3 files, don ' t contain content that can be indexed. As a result, the search indexing servers in Exchange and SharePoint don ' t perform full-text indexing on these types of files.
These types of files are considered to be unsupported file types.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/partially-indexed-items-in-content-search?
view=o365-worldwide
https://docs.microsoft.com/en-us/office365/securitycompliance/export-a-content-search-report

This is not a permissions issue.
The on-premises Active Directory domain is named contoso.com. To enable users to sign on using a different UPN (different domain), you need to add the domain to Microsoft 365 as a custom domain.

Deploy Microsoft Entra ID Pass-through Authentication
Step 1: Check the prerequisites
Ensure that the following prerequisites are in place.
In the Entra admin center
1. Create a cloud-only Hybrid Identity Administrator account or a Hybrid Identity administrator account on your Microsoft Entra tenant. This way, you can manage the configuration of your tenant should your on- premises services fail or become unavailable.
(E) 2. Add one or more custom domain names to your Microsoft Entra tenant. Your users can sign in with one of these domain names.
(A) In your on-premises environment
1. Identify a server running Windows Server 2016 or later to run Microsoft Entra Connect Sync. If not enabled already, enable TLS 1.2 on the server. Add the server to the same Active Directory forest as the users whose passwords you need to validate. It should be noted that installation of Pass-Through Authentication agent on Windows Server Core versions is not supported.
2. Install the latest version of Microsoft Entra Connect Sync on the server identified in the preceding step. If you already have Microsoft Entra Connect Sync running, ensure that the version is supported.
3. Identify one or more additional servers (running Windows Server 2016 or later, with TLS 1.2 enabled) where you can run standalone Authentication Agents. These additional servers are needed to ensure the high availability of requests to sign in. Add the servers to the same Active Directory forest as the users whose passwords you need to validate.
4. Etc.
(B) Step 2: Enable the feature
Enable Pass-through Authentication through Microsoft Entra Connect Sync.
If you ' re installing Microsoft Entra Connect Sync for the first time, choose the custom installation path. At the User sign-in page, choose Pass-through Authentication as the Sign On method. On successful completion, a Pass-through Authentication Agent is installed on the same server as Microsoft Entra Connect Sync. In addition, the Pass-through Authentication feature is enabled on your tenant.
Incorrect:
Not C: From Active Directory Domains and Trusts, add a UPN suffix
Not D. Modify the email address attribute for each user account.
Not F. Modify the User logon name for each user account.
Reference:
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/connect/how-to-connect-pta-quick-start

Explanation:
Limit a user named User 1 from sending more than 30 email messages per day: Anti-spam policy The anti-spam policy in Microsoft Defender for Office 365 allows you to configure outbound spam settings, including limiting the number of email messages a user can send per day. This helps prevent spam and potential email abuse within the organization.
Prevent the delivery of a specific file based on the file hash: Safe Attachments policy The Safe Attachments policy in Microsoft Defender for Office 365 can be configured to block or quarantine emails with attachments that match specific file hashes. This ensures that potentially malicious files are not delivered to users' inboxes.