Dear Amin El-Zein,
Good day!
Many thanks for posting back here,
For the question "can i do it for each mailbox? or it's a general one?"
Yes, you're right, this setting is at the organization (tenant) level. However, if you want to set for each mailbox, you can create one new policy in Exchange Admin center--> set MaxEmailAgeFilter in the new policy --> assign this policy to the mailbox. You can refer to the following steps and screenshot.
Step1: Create a new policy, Go to Exchange Admin Center --> Mobile --> Mobile device mailbox policy -->New. Fill the fields, then Save.

Step2: Set the Mobile device mailbox policy for newly created policy using PowerShell command.
Set-MobileDeviceMailboxPolicy -Identity Test(the policy that you created in exchange admin center) -MaxEmailAgeFilter ThreeDays
Step3: Check if the newly created policy modified.
Get-MobileDeviceMailboxPolicy -Identity Test
Step3: Assign the newly created and set mobile device mailbox policy for specific mailbox.
**Set-CASMailbox -Identity ****@domain.com(your mailbox) -ActiveSyncMailboxPolicy "Test"
For additional information, you can refer to Add or remove users from a mobile mailbox policy: Exchange 2013 Help | Microsoft Learn
We look forward to your update. Please feel free to let us know if there are any questions or if we've got you wrong. We will keep assisting you based on the information you provide. We sincerely appreciate your patience and cooperation.
Thanks for your precious time.
Have a nice day!
Sincerely,
Libeamlak | Microsoft Community Moderator