Hi Tarjani,
1. Batch update their personal settings for existing users.
We will use XrmToolBox's "User Settings Utility" tool.
This is the download page of XrmToolBox: https://www.xrmtoolbox.com/
After downloading and unzipping it, open XrmToolBox.exe.
First connect to your D365 organization and log in with your account and password.
Then install the User Settings Utility in the tool library and open it.
Click Load Users and settings, Check all, select Allow emails on my behalf to Yes, finally Update Users Settings.

In this way, the personal settings of all users are updated.
2. Automatically update the personal settings of new users.
We will use Power Automate.
Log in to https://asia.flow.microsoft.com/, select the D365 environment.
Create a Power Automate with “Automation - Blank”

Add “When a record is created” card of the CDS connector and set the Entity Name as “Users”:

Add “Update a record” card of the CDS connector and set the Entity Name as “User Settings” (the second one) and Record identifier as “User” which is the User GUID:

Find IsSendAsAllowed and select it to Yes.

Save this flow.
In this way, the personal settings of the new users are also automatically updated.