When you send a time sensitive message, set an expiration date so your recipients won’t waste time responding after the deadline has passed.
In the More Options group, click the Message Options Dialog Box Launcher .
Under Delivery options, check Expires after, and then select a date and time.
When the expiration date passes, the message remains visible in the message list with a strikethrough. It can still be opened, but the strikethrough gives a visual clue that the message is out of date or no longer relevant.