Barbara Jensen's mail is on the b.imap.itd.umich.edu server. The umich.edu machine opens a connection to b.imap.itd.umich.edu and transfers the message. The b.imap.itd.umich.edu machine then looks up the location of Barbara's mailbox and puts the message there for her. Then it's up to Barbara to decide when to check her e-mail and whether she wants to look at the message. Occasional delays, however, are sometimes unavoidable.
Any time there's more than one step to a process, there's more than one place things can go wrong.
The most obvious problem is a power failure. A brief interruption of power may have no effect. Sometimes network cables are inadvertently cut. Human Error People make mistakes with software as well as with power and cable lines. Here on campus this September, a software bug caused a mail loop that sent duplicates of messages to people and clogged e-mail servers.