Office 365 enable in place archive and set retention tags to archive emails over 1 year old

If a mailbox is approaching 50Gb and  you want to archive old emails to an in place archive do the following:

• From the office 365 portal go to Exchange admin.
• Under recipients -> mailboxes (or shared for a shared mailbox) open the mailbox
• On the right go to Manage mailbox archive
• Enable archive

• In User Mailbox, click Mailbox features.
• In the Retention policy list, select the policy you want to apply to the mailbox, and then click Save.
• This will provide a list of archive or delete policies the user can then choose from
• Login to the mailbox
• In OWA right click on the folder, choose Assign policy and pick the archive policy to apply
• By default sub folders will use the parent policy, you can set the policy per folder
• You will need to seperately set the tag on the inbox, sent items and any other parent level folders

After you apply the new retention policy to mailboxes it can take up to 7 days in Exchange Online for the new retention settings to be applied to the mailboxes. This is because a process called the Managed Folder Assistant processes mailboxes at least once every 7 days. Instead of waiting for the Managed Folder Assistant to run, you can force this to happen by running the Start-ManagedFolderAssistant cmdlet in Exchange Online PowerShell.

Also see:

https://docs.microsoft.com/en-us/microsoft-365/compliance/set-up-an-archive-and-deletion-policy-for-mailboxes

https://docs.microsoft.com/en-us/exchange/security-and-compliance/modify-archive-policies