That is, after two days, undeliverable mail is returned to the sender. However, for many years, email users have received and expected much faster delivery. It was rare for a local message to reach an on-campus destination in more than an hour. During the recent crisis, people were upset that it was taking 4-6 hours for email transmission. In some instances, it took a day.
Some situations come up that are completely out of local control.
For example, a number of departmental systems on campus have frequent periods when they do not accept electronic mail delivery. When mail is sent to a user on such a system, delays in email delivery may occur. IT does not control those systems. Sometimes, an earlier message can arrive after a later message sent to the same person. IT has no control over electronic mail once it has left campus.