Configuring POP3 downloading

We recommend using SMTP. This is the proper protocol for receiving mail. If you have a continuous line or dial on demand router, use SMTP. POP3 was meant only for e-mail clients, not for mail servers to retrieve mail.

However, in some cases you might not have a choice and you have to use POP3 to download your mail.

Using POP3 to receive mail

Post office protocol (POP3 (RFC 1225)) is a client/server protocol for storing email so that the client can connect to the POP3 server at any time and read the email. A mail client will make a TCP/IP connection with the server and by exchanging a series of commands, read the email. All ISP's support POP3.

Advantages of using POP3 to retrieve mail

Simple

Any ISP can support it

No need for fixed IP address.

Disadvantages

BCC messages are not routed within your organization.

If you use a POP3 mailbox for each user, you have to create mailboxes twice - once at the ISP and once on Exchange server.

If you use one POP3 mailbox for multiple users, messages send by list servers are not always routed correctly. If your ISP mail server does not support the `for' clause, messages from some mailing lists will not be routed. This is because when mail is sent via SMTP, the actual recipient is provided by the sender on the "RCPT" command. This information is called part of the "envelope" (since it is outside of the message), and is sometimes not included in the actual mail message's header. For a single recipient, this is not a problem. If the mail is in your mailbox, you know it is for you. However, if all mail directed at a specific domain goes into the same mailbox, there may be no way of determining who the mail should be delivered to. This is most often the case for messages from mailing lists or if the BCC: field was used. There is however a solution for this problem. The most common is in the Received: line. According to page 32 of RFC 821, the Received: line should look something like this:

Received: from sender.com by yourisp.com for you@yourdomain.com

The "for" clause is derived directly from the envelope information, so even if the To: and Cc: lines make no mention of "you@yourdomain.com", the true recipient can be found here. Thus, any POP to Exchange solution must (at least) be able to parse the Received: lines in the header in order to forward the mail to the correct local recipient.

Note: An easy way around the above problem is to create dedicated POP3 mailboxes for lists. Then route the lists to a public mailbox, so that other users can also benefit from the lists.

Using SMTP to receive mail

Simple Mail Transport Protocol (SMTP(RFC821)) is a server-to-server protocol for sending e-mail across the Internet. Briefly, a mail client will make a TCP connection to an ISP's SMTP server and upload a mail message (complete with headers) and instructions to whom the message should be delivered. The SMTP server will then either deliver the message (if it knows the final recipient) or pass it along to another SMTP server. SMTP works best when all servers are connected all the time. If the receiving server is not available, then the sender will have to queue the message and try later. Eventually, the sender will either make it through or give up and return the message to its originator. In the case of dial-up connections, the receiver may be unavailable more often than not.