enable mailbox archival

Hi, Rising Flight

From your description, I understand that you want to turn on archiving for your mailbox. Keep messages in your mailbox for a year and move the rest to your archive mailbox.

We can first enable the archive for the mailbox, then create the retention tag. Subsequently, add the retention tag to the retention policy, and then apply the retention policy to the mailbox.

Here are some steps for reference:

1.Enable the archive mailbox for the user. More information can be found Enable archive mailboxes for Microsoft 365 | Microsoft Learn

2.Create new retention tags for the archive. More information can be found Customize an archive and deletion policy (MRM) for mailboxes | Microsoft Learn

365 can be selected for retention period. This is shown in the figure below.

3.Create a new retention policy. Add retention tag to Retention policy. This is shown in the figure below.

4.Assign the new retention policy to user mailboxes. In the EAC, select the mailbox with archiving enabled, and under the Mailbox option, select Manage Mailbox Policy to add the retention policy.

If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.