Retention Hold is a feature in Microsoft Exchange Online. Once Retention Hold is applied on a mailbox all the Retention Tags for the Retention Policy are not processed. This essentially means that Items will not expire or get deleted. The use or need of a Retention Policy is to manage emails in a way that the Inbox doesn’t become too full.
A Retention Policy consist of Retention Tags. And a Retention Tag can be used to tag default folders in a Mailbox such as Inbox, Sent Folder etc. Default Folders are folders which are created automatically once a Mailbox is created. So with Retention Tags we can specify an Action and a duration after which the Action would take place. For Example we could have a Retention Tag on the Inbox which says that Delete all Emails which are older than 2 years. In this case the Action is Delete Emails and the Duration is 2 years. The reason why it is named as “retention” is because Mails would be retained in the Inbox for a specified period of time after which an Action would take place either Deleting the Items or moving it to another folder or an Archive Mailbox.
This is a good way to manage your Mailbox. Following this Principle Emails would not accumulate and would periodically get deleted. In case you still need old emails, you could create an Archive Mailbox and create a Retention Action which specifies to Move Emails from the Inbox to the Archive Folder after a specific period of time. This can be done by using a Default Policy Tag.
Now, suppose you have some people who are on a vacation. It is more than likely they would not be checking their emails. Now what if there is a Retention Policy which Deletes Emails in the Inbox every 2 weeks? This means that the lifespan for an Email is only 2 weeks after which it would be deleted. So the people on vacation would actually be missing all the communication once they come back. Microsoft has a solution for this situation. There is something called as a Retention Hold.
Once a Retention Hold is applied on a Mailbox, the Retention Policies and the Policy Tags do not get processed. Due to this the Emails in the Inbox would not have an expiry date and would continue to remain in the Inbox even after 2 weeks. So if Employees are on Vacation, the Exchange Admin normally puts their Mailbox on a “Retention Hold”. All Emails are Retained so that the people can access them once they come back. And then the admin can remove the Retention Hold from the Mailbox.