How can I mark a message, similar to flagging a message, as “irrelevant” after a certain date?
For this Outlook has the “Expires after” message option. When the set date is reached, the message will appear in a Grey strike through font for easy recognition.
Setting the expiration date
You can set an expiration date on a message, in the Message Options dialog. For this you’ll first have to open the message by double clicking it. You can then find the Message Options dialog in the following way;
In this dialog you can select the “Expires after” option and set the date and time after which the message would be no longer relevant to you.