Queues make disconnected networks possible. A queue is a temporary storage place for network messages. Since the Internet is a heterogeneous, disconnected collection of computers each operating with undefined reliability characteristics, a fundamental networking problem has to be solved: how can a message get from A to B when both A and B aren't up and running at the same time?
Nowhere is this more needed than with email. Unlike a telephone call, where the caller and the receiver are both online, an email message can be sent when the receiver is not online, and the same message can be received when the sender is not online. The infrastructure between the sender and the receiver is the queue. The Internet standards that govern this queue are SMTP, POP and IMAP; while some of the open source software that uses these protocols include Sendmail, qmail and dovecot.