What is the definitive, exhaustive list of fields searched by the Microsoft Graph API /messages?$search parameter?

I am using the Microsoft Graph API to allow users to search for emails within a specific mail folder. I am using the $search query parameter on the /users/{id}/mailFolders/{id}/messages endpoint, which works well for general keyword searches.

Here is a simplified example of my C# code using the Microsoft Graph SDK:

// C# using Microsoft.Graph SDK v5
var searchKeyword = "project budget";

var messages = await graphClient
    .Users["user@example.com"]
    .MailFolders["folderId"]
    .Messages
    .GetAsync((config) =>
    {
        config.QueryParameters.Search = searchKeyword;
        config.QueryParameters.Top = 25;
        // I also select specific properties I need
        config.QueryParameters.Select = new string[] { "subject", "from", "receivedDateTime" };
    });

What I know so far:

The $search parameter is designed to function like the main search bar in Outlook, providing a user-friendly search across multiple fields simultaneously. Based on the official documentation and various community sources, I understand that the search targets at least the following properties:

• subject
• body
• from
• toRecipients, ccRecipients, bccRecipients
• The content of attachments (for supported file types like Office documents, PDFs, etc.)

My Question:

While this is useful, I am looking for a definitive and exhaustive list of all properties that are indexed and included by default in a simple $search="keyword" query.

My goal is to have a complete technical understanding so I can accurately document the feature for my users and stakeholders.

Specifically:

1. Is there an official, published, and exhaustive list of all default fields targeted by the $search parameter on the /messages endpoint?
2. Does this default set of fields include other metadata, for example, categories, specific Internet Message Headers, or other extended properties?
3. I understand that this search functionality is powered by the underlying Exchange Online search service. Has the full list of indexed properties for this specific API use case been documented anywhere in that context?

I am aware that I can use Keyword Query Language (KQL) to explicitly target specific fields (e.g., subject:budget), but my question is specifically about the default behavior of the $search parameter when no fields are specified.

Thank you for any clarification you can provide.

When no search keyword is specified, the search is carried out on the default search properties of from, subject, and body.

The table below shows an overview of the message properties supporting searching. During the searching you need to use a specific keyword related to each property.

Check more in my article