Archiving a Conversation
From the Messages screen, select the message you want to archive and select the information icon to open the Conversation Details pane
Select Archive and confirm your selection.
Unlike voicemail boxes, each subscribed user maintains their own read state and archived state, so if one user archives a conversation it will still display for other users who have not archived it.
Unarchiving an Archived Conversation
Archived conversations will automatically unarchive if the conversation receives a new message or if you send a new message to the same list of recipients as was in the archived conversation.