Whenever you claim a reward for completing various Missions, clearing dungeons, unlocking Achievements, etc. they’ll be sent to your Inbox for redemption most of the time. Items in your inbox will only remain for at least a week; unclaimed items in the inbox will expire and will be lost forever. It’s alright to use the inbox as a temporary storage in case your hero/rune/badge slots are maximized; just remember to clear it regularly to avoid wasting the valuable items inside.