When activity for a mail is complete, you should close it out so you can view and report on mail which is still active.
Close out project mail to get accurate project status reports. You can close out any project mail sent by your organization, but typically, you'd close out formal communications such as RFIs.
Closing out mail also removes any tasks associated with that communication from the task lists of the mail's recipients. Before you close out a mail, make sure its associated tasks have been completed.
You can only close out mail that was sent by your organization. If you need to close out other organization's mail, either respond to the mail to clear the task or ask them to close it out.
Mail can also be closed out from the mail thread view, but you must be a user of the sending organization to see the ‘Mark as Closed-Out’ button.
You can close out an individual mail or the entire thread. The entire thread is closed out automatically when the parent mail (first mail in the thread) is closed out.
Accidentally closed out a mail? You won't be able to undo the action, but as a workaround, you can forward that mail to the same recipients. The new mail will retain the same reference number, allowing you to continue the conversation within the thread.