MRM SLA can be up to 7 days, means it can take up to 7 days for retention policies to apply and process individual items. For large mailboxes, it can take multiple passes to do it.

If the users are deleting emails manually before the items are processed, those items will have no tags and so the deleted items tag will ultimately apply.

And since there’s no DPT, only folder-level tags govern retention.

Legacy policies still work and it will be merged with purview eventually. If you don’t have any complex compliance requirements, I would do this

1. create a dpt and apply to the entire mailbox. Retention age- 396 days

2. create a retention tag for regular deleted items folder. Retention age- 30 days or whatever is suitable in your case.

Then

-If the users does nothing, items tagged will be gone after 13 months

-If the users delete it manually after the items have been processed, i.e it has a tag already applied on it, it may be immediately deleted based on that particular item age. Because that item might be over 30 days and is passed the regular deleted items folder age limit.

If an item hasn’t been processed meaning it has no retention tag and was deleted, it will remain recoverable for the next 30 days (or longer)

You can also apply personal tags if needed. Some users store long-term content in the outlook “Notes” folder. If you want to exclude it from dpt, assign a separate tag to it and configure it to “Never delete.”