See explanation below.
Explanation
1. Navigate to the Exchange Admin Center
2. Navigate to Compliance management > Retention tags, and then click Add +
3. Select the Applied automatically to entire mailbox (default) option.
4. The New retention tag page title and options will vary depending on the type of tag you selected. Complete the following fields:
Name: Enter a name for the retention tag.
Retention action: Select Delete and Allow Recovery option.
Retention period: Select When the item reaches the following age (in days) option.
Comment: User this optional field to enter any administrative notes or comments. The field isn't displayed to users.
5. Navigate to Compliance management > Retention policies, and then click Add +
6. In New Retention Policy, complete the following fields:
Name: Enter a name for the retention policy.
Retention tags: Click Add + to select the tags you want to add to this retention policy.
After you create a retention policy, you must apply it.
1. Navigate to Recipients > Mailboxes.
2. In the list view, select the mailbox to which you want to apply the retention policy, and then click Edit.
3. In User Mailbox, click Mailbox features.
4. In the Retention policy list, select the policy you want to apply to the mailbox, and then click Save.
Reference:
https://docs.microsoft.com/en-us/exchange/security-and-compliance/messaging-records-management/create-a-re
https://docs.microsoft.com/en-us/exchange/security-and-compliance/messaging-records-management/apply-reten