POP vs IMAP for Inboxes

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Client-server email is still useful even in this day of massive public webmail. This note is for people interested in the POP and IMAP protocols used to access client-server email. There is an enduring myth that there is some circumstance in which POP is better than IMAP, whether for the user or the provider. So here's the answer!

It was Portia Shao of Innosoft (a VMS company) who first made me think about POP's problems in the early 1990s. I remember her email signature today: POP3's "Leave mail on server" is for the birds.

After thinking about that I realised POP's shortcomings, which led me to find the in-development IMAP protocol, which, after years of pointless political fighting from the University of Washington, became the IMAP protocol we have today.

Dan 05:32, 9 October 2006 (CST)

The Myth and the Failure

Leaving a POP-accessible inbox on the server doesn't scale. The POP3 protocol is very simple-minded, and if you use its facility for leaving mail on the server, your POP3 client will fetch the entire contents of the inbox every time you connect. As you add even just a few hundred users, 9 o'clock in the morning becomes the time the mail server grinds to a halt! But with POP, that's the only way you can read your inbox from multiple computers. So of course nobody uses that.

But there is a use for POP that people still recommend. Even now, in 2006! I came across it the other day:

Ah ha! they say. POP is an old but efficient protocol that is ideal in this one case: where you have a mail server and want to conserve resources on the server by forcing users to download all their inbox every time they connect, after which it is deleted.

No! No! No!

IMAP For Ever

Start by reading the old but excellent short paper http://www.imap.org/papers/imap.vs.pop.brief.html . You learn here about online and offline modes of client-server email, and that both POP and IMAP can be used in offline mode. There are many IMAP daemons and some are more efficient than others, but there is nothing in the protocol that makes offline IMAP worse than POP, which can only be offline.

Even better, IMAP is superior in offline mode to POP due to status flags that allow better error detection and correction. With offline IMAP you can be sure the inbox downloaded completely. The high-quality free software IMAP server Dovecot fully supports this capability.

The beauty of using IMAP for offline mode is that you always have the option to switch on other features over time. IMAP does folders (including nested and public), a form of ACL and more. If you have a POP-based infrastructure you can't give any of your users access to these without adding new software at both the client and the server end. If you use IMAP to begin with, you can just enable some or all extra features for some or all users.

Thankyou, and goodnight.

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