E-mail is often received via a different server than the one that sends e-mail.
The type of server depends on which type of e-mail tool you use. For those
using an e-mail client, your e-mail is probably delivered to you via the most
common method, Post Office Protocol 3 (POP3) server. (We have no idea what
happened to the first two.) The POP3 server receives all its e-mails from SMTP
servers and sorts them into file spaces dedicated to each user (much the same
way mail is put into post office boxes at a local post office—thus the name).
When you open your e-mail client, it contacts the POP3 server to request all
the new e-mails. The e-mails are then transferred to your PC, and in most
cases the e-mails are erased from the POP3 server.
Another common method (or protocol) for mail retrieval is an Internet Mail
Access Protocol (IMAP) server. This is the protocol normally used by webbased
e-mail clients, and corporate e-mail systems such as Microsoft
Exchange. The IMAP server receives and sorts e-mail in much the same way as
a POP3 server. Unlike POP3, however, IMAP does not transfer the e-mails to
the machine of the account holder; instead, it keeps e-mail on the server. This
allows users to connect to use their e-mail account from multiple machines.
IMAP also allows for server-side filtering, a method of presorting e-mail based
on rules before it even gets to your PC. It’s kind of like having a friendly
postal worker who sorts all your bills to the top and magazines to the bottom.
Two main issues with IMAP servers are storage space and working offline.
Most Internet e-mail services put a limit on the amount of storage each subscriber
gets (some charge extra for additional storage space). In addition, these
services often limit the file size of attachments (such as photos). The other
issue is the ability to work offline or when not connected to the Internet. One
solution is called caching, which temporarily places the subscriber’s e-mail
information on whatever PC he or she wants to work offline with. When the
user reconnects, any e-mails created while offline are sent, and any new incoming
e-mails can be viewed.