This disclosure is directed to automatically moving message objects, such as email message objects, from online source mailbox databases to online archive mailbox databases. As described herein, each email message in a source mailbox database is directly or indirectly associated with a retention policy tag. The retention policy tag associated with a message object in a source mailbox database specifies a retention period. The message object remains in source mailbox database until the retention period expires for the message object. When the retention period for the message object expires, the message object is automatically moved to an archive mailbox database.