Deny entry to any email from an non-existing domain. Unlike postal mail, which is delivered to your mailbox prepaid by the sender, sometimes you have to pay to check your email. You may pay telephone charges. This is especially frustrating when you are under time constraints, as you are when changing planes. Computer resources are required to process the junk mail.
Junk mail uses bandwidth.
Junk mail uses CPU time. Junk mail uses disk space. There are an increasing number of cases where the amount of junk mail has caused users to exceed their disk quota, so that subsequent real mail was turned away and not received. The recipient's time is required to sort out the junk mail. As the quantity of junk mail increases, the cost of doing this sorting can become quite significant.