What makes it possible for e-mail to be sent and received on a wide variety of devices, including cell phones, PDAs, laptops, and desktop computers?

Compatibility.

Explanation:

Email or electronic mail is a messaging system that allows for transmission of digital or electronic mails to computers within and to other networks through the internet.

Email applications are web based applications and are hosted on servers. The POP (post office protocol) and IMAP (internet messaging access protocol) are used to message the mails sent to a client on a server. The application is written with standards and protocols to ensure compatibility in all types of device form factor, so an email can be view in a laptops, smartphones etc.