Disabling a users is the safest Option
If you’re having trouble remember the correct usage of these tasks think about it in terms of the Exchange Management Shell commands and how they pair up.
<![if !supportLists]>· <![endif]>Enable-Mailbox is for creating a mailbox for an existing user, and Disable-Mailbox removes only the Exchange attributes from the user account
<![if !supportLists]>· <![endif]>New-Mailbox is for creating a new AD user and mailbox at the same time, and Remove-Mailbox removes both the mailbox and the user object.
But if thats too hard just to try to remember that Disable is the safest option, because you can quickly reconnect the mailbox to the user again if it is still within the retention period.