Valuable processing time is now allocated to determine where to deliver the incoming mail; this causes other real applications on the server to slow down because they have to share the processor with other tasks.  Disk space on the server must be allocated to store these unsolicited messages until they are read and deleted.  Low available disk space on a server can cause the server to run very slowly or even crash.  Backups run longer and may require more than one tape to complete daily backups.