IMAP vs POP3
IMAP and POP3 are the two most popular e-mail protocols.
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[edit] Approach to Handling E-mail
POP3's approach to e-mail is to download e-mail off of e-mail servers for client storage and use.
IMAP's basic approach is to keep the e-mails consolidated in the server and have clients read data off of the server.
[edit] POP3's Benefits
POP3's benefits include less bandwidth usage, offline e-mail access, and less use of server storage.
On the other side the server storage reason is not really true. It's the default value of most email clients. But there is no client (or give us proof!) which doesn't support an option to download new mail and keep it on the server.
However, server storage is generally reduced because sent mail isn't usually stored server-side (unless a savvy user copies it back).
[edit] IMAP's Benefits
IMAP's benefits include e-mail synchronization between many clients and less use of client storage. Also, while POP3 can be used to store mail centrally (with the "leave on server" option), it has no provision for creating server-side mail folders. IMAP can index mail server-side, and server-side mail searching and sorting is provided in the protocol. As stated, POP3 can leave mails on the server, but there's also the option in some clients to not store mails on the server. Thus, if you have several mail clients, say on different computers, or also on a webclient, mails may not be in sync (consider tech-inept clients working with your mail server, not you). With IMAP, mail storage on the server is a key feature. Did we mention, that sometimes client's harddrives may break down? Mail storage on the server, possibly in connection with a local caching IMAP client is the state-of-the art solution for serious e-mail providers. (You DO make backups on your server, don't you?)
[edit] Cached IMAP
Many IMAP clients support cached IMAP, which caches e-mails on the client and downloads only new e-mails. This has the benefit of using less bandwidth (and allowing users to quickly fetch their e-mails from local storage). The disadvantage is that e-mail is stored both on client and server, thereby doubling the net storage use.
[edit] Server Performance
[edit] Timeout
POP3 will close "dead" connections quick
IMAP timeout should be 30min. Processes are blocked due of this RFC part.
[edit] Usage of Serverprocessess
POP3 connections will be closed directly after downloading. Not much processes needed.
IMAP connections will stay open till the client is finished. Processes are open till the user is finished. Results in much process on the server side (and thus more usage of RAM)
