1 March 1994
Debbie Wong (postmaster@ualberta.ca)
CNS System Software
The Post Office Protocol (POP) defines how unfiled e-mail is down- loaded from a mail server to a client mail program. Filed mail is handled entirely by the client machine's local file system. Mail is sent by the client, using the Simple Mail Transfer Protocol (SMTP), to an Internet host which in turn forwards it on. POP client programs include NuPOP and Pegasus Mail for DOS, and Eudora for the Macintosh and Windows. While POP clients also exist in the UNIX world, running them is somewhat redundant in our campus environment since most UNIX workstations run their own Message Transfer Agents (MTAs). POP servers that are important in our environment include any UNIX machine and Novell file servers running the Mercury software.
CNS supports central SMTP and POP connectivity to all logon hosts, and SMTP capable MTAs and POP servers. POP clients are available on the CNS anonymous FTP server. POP connections over serial lines exist, but CNS does not support them.
POP as your primary mail protocol works best when you have one main client machine from which you handle your mail. While you have the option of checking unfiled mail from other machines while leaving a copy on your mail server, at some time you must download it to where your filed mail is kept. A POP solution is best for you if your department does not provide servers and if your working machine is on the Internet but is not itself a logon host. (There are currently about 40 users of POP on the CNS Logon Server, and about 100 other users on ten other campus departments' POP servers.)
The strengths of POP clients lie in their system-specific user interfaces and their different connectivity options (Eudora in particular is quite a good client). Filed mail, private address books, and private mailing lists are defined on the user's per- sonal computer, so all can be accessed within the environment the user knows best. Unfiled mail can be read from anywhere on the Internet, or from home. The POP protocol provides support for the Multipurpose Internet Mail Extensions (MIME) which allow the user to attach binary files, such as formatted word processor documents, to e-mail messages.
Because filed e-mail is manipulated using native file system calls on a desktop machine, it is impractical to work with filed mail except from the machine that you normally use. Given POP's strengths in connectivity, however, one viable alternative is to use a portable computer that travels with you wherever you go. In addition, those users who do not need access to filed mail except at work, but who want access to unfiled mail from anywhere, are well served by POP protocols.
Due to the nature of POP, unfiled mail is usually downloaded to the client machine without pre-selection. This is not effective since large files may be downloaded unnecessarily, particularly in the case of MIME binary files. This can be avoided by setting a switch which tells the client program to download/or not to not download any large files.
Correct configuration of the user's e-mail address is not always easy with POP clients. There are also no effective ways a local administrator can provide value-added services to the e-mail environment.
The Interactive Mail Access Protocol (IMAP), a superset of POP, is essentially a new standard that will provide more functionality in the future to POP client software. IMAP addresses most of the shortcomings of POP. For example, IMAP defines how a client program asks a mail server to present a list of interesting mail and how mail can be selectively downloaded to the client. Inter- esting mail can be defined by status (unfiled and filed), con- tents, location, and header envelope. When interesting is defined to be unfiled, and all unfiled mail is downloaded, IMAP emulates POP. Because of the selective nature of IMAP, network resources are used efficiently, a matter of special concern in the age of MIME.
Unfortunately, there are currently no good IMAP clients for any platform besides UNIX (where they are somewhat redundant), so the benfits of IMAP are still in the future. Any UNIX machine can be a potential IMAP server.