There are currently three main Internet Email Protocols, SMTP, POP3, and IMAP4. Also there is ODMR which is a variation of SMTP. (There are several other mail protocols, eg UUCP which are currently less widely used, and aren't currently supported by VPOP3). IMAP4 is only supported by VPOP3 Enterprise, not by VPOP3 Standard.
SMTP (Simple Mail Transfer Protocol) is really intended for permanent connections to the Internet. The SMTP 'client' connects to an SMTP 'server' to send a message. There is no way to request a specific message using SMTP, but there are extensions to request a server to start sending any messages it has.
When messages are sent using SMTP it is sent in two parts:
1) An Envelope - this contains the email address it was sent from (typically for error reports) and a list of people to receive the message. This is not normally seen by users.
2) The message Data - this contains the message that you typically see.
The Envelope may contain a copy of the information in the Data's From: and To: header fields, but it may contain other information which is not contained in the message at all (for instance, for mailing list messages or if BCC addressing is used).
If you have an SMTP account with your Internet Provider, then you need to run an SMTP 'server' (e.g. VPOP3) on your PC, and then somehow tell your Internet Provider to start sending messages to it after you've connected. Some Internet Providers can automatically detect you dialing into them, and immediately start sending mail to your SMTP server. Other Internet Providers need you to issue an SMTP command such as ETRN to their server to trigger mail delivery.
A few Internet Providers require a non-standard action, such as sending a 'Finger' command to their server to trigger mail delivery. You can use an external program, set as the VPOP3 Post-Connection Extension to issue this command.
Once SMTP mail delivery has started there is really no way for the server to reject messages based on any other criteria apart from the data contained in the Envelope (i.e. From or To addresses). Also, once the message has been sent to an SMTP server, the client typically discards its copy.
This means that if VPOP3 is running as your SMTP server to receive mail from your Internet Provider, some useful features such as being able to limit message download size, the Download Rules, leaving messages on the ISP server etc, cannot be used. VPOP3's SMTP Rules can perform many actions you may require, but they are not as flexible as Download Rules, because VPOP3 cannot see the message header without receiving the entire message.
When sending messages to another site, there are really two ways of doing this, both of which are typically accepted:
The mail server can send the message directly to the destination site's mail server
The mail server can send the message to a relay-server which then sends the message to the destination site.
We recommend the use of the second method. Some of the reasons for this are:
It is easier to configure, and fits in with most users' understanding of how mail works.
It is a lot quicker over a dial-up connection. If the first method is used, then any message to more than one recipient typically has to be sent multiple times (once for each recipient). There is also a lot of querying of DNS servers which can be time consuming.
Many Internet Providers (eg AOL) will reject mail that comes directly from a computer on a dial-up connection, as an anti-spam protection method.
The ODMR (On Demand Mail Relay) protocol is a variation on SMTP which has been designed to allow SMTP mail delivery to a dynamic IP address. With ODMR, the ODMR client (VPOP3) connects to an ODMR server (at the ISP), logs on with the ISP account details, and from then on acts identically to an SMTP server (so the ODMR client becomes the SMTP server, and the ODMR server becomes the SMTP client).
This allows the advantages of SMTP without requiring a static IP address.
POP3 (Post Office Protocol Version 3) was created for dial-up Internet accounts because of the limitations with the SMTP protocol. When collecting mail from an ISP using POP3, the 'client' is the PC at the user's end, and it is in total control of what messages it receives and which ones it doesn't.
The POP3 client can also typically view message headers without downloading the entire message, see the message size before downloading it, delete messages without downloading them, or leave messages on the server after downloading them. Because of these POP3 facilities, VPOP3 can do a lot more to help you.
One of the normal problems which occurs if multiple email addresses are directed to a single POP3 mailbox is that the SMTP Envelope (see above) is lost when the message is placed in the mailbox. This means that the explicit message routing information is lost, and all that VPOP3 has to go on is the data in the message headers (eg To, Cc etc). This can cause problems if you receive messages from mailing lists or which include Bccs. Some ISPs get around these problems by extending the POP3 protocol (eg Demon Internet Services do this) or adding special message header fields which VPOP3 can use if you tell it about them.
See here for information on the IMAP4 protocol.