Disable: will remove the Exchange attributes for the user account but leave the user account in Active Directory. The mailbox is then also left in the Exchange mailbox database until the retention period (30 Days) has passed, then it is removed permanently.

Remove: will remove both the Exchange mailbox and the user account from Active Directory.

Note: Disable is the safest option, because you can quickly reconnect the mailbox to the user again if it is still within the retention period.

i. Go to Exchange Management Console >> Microsoft Exchange >> Microsoft Exchange On Premises >> Recipient Configuration:
ii. Right Click on Mailbox, you will see two options, Disable or Remove.