POP MailboxesIntroductionThere are a number of distinct concepts related to email services, and it is easy to get them confused. One of the concepts is that of the POP mailbox. This page attempts to explain what POP mailboxes are and how they fit into the overall framework of email. Note: There is a glossary at the end of this page.
What Is A POP Mailbox?A POP mailbox is a place where email messages are held for you until you get around to "checking your email". When someone sends you an email message ("an email"), the email is routed to your POP mailbox where the email is stored until you "check your mail". Each POP mailbox resides on a single computer and has an identity that consists of the name of the computer (e.g. pop.sooperdooperisp.com) and a pop mailbox name (e.g. dave). Each POP mailbox also has a password to prevent unauthorized access. When you "check your email", your email client
(e.g. Eudora or Outlook) connects to a particular POP server
and provides your username and password and retrieves your
emails from your POP mailbox. Usually, your email client
will instruct the POP server to delete the email in your POP
mailbox after the email client has read the email from the
POP mailbox.
Why It's ConfusingWhen you sign up for an "internet account" with an ISP (Internet Service Provider), the ISP usually provides you with three things: A Dialup Account: This facility allows you to establish an IP connection with the internet. You will be given a telephone number to dial up along with a username and password. To dialup, you typically enter this information into your PPP connection program. This is the only facility you need from an ISP in order to access any service on the internet (e.g. web, news). An Email Address: The email address that an ISP provides you will usually be in the ISP's namespace. For example, if your name is Dave and your ISP is called Sooper Dooper ISP, then your email address will probably be something like dave56@sooperdooperisp.com. The ISP configures its mailserver so that all email addressed to that email address ends up in your POP mailbox (see below). A POP Mailbox: This is a place to store email that arrives for you at the email address your ISP has created for you. Your POP mailbox will reside on a particular computer at the ISP (e.g. pop.sooperdooper.com) and will have its own name (sometimes called "username") and password. This is a very clean organization because all of the separate services that the ISP is providing to you are defined separately and independently of each other. For example, the username and password on your dialup account can be entirely independent and different from the username and password on your POP mailbox. What makes it confusing is that, to avoid confusion,
many ISPs make the username and password for the dialup
account they provide for you the same as the username and
password for the POP mailbox they provide for you. This
single username/password pair is presented to you as
"your internet username and password" and as the
customer usually associates this with dialup, the POP
mailbox concept simply vanishes in the customer's mind. As a
result of this, most internet users do not know what a POP
mailbox is, and are unaware that they even have one.
Separating Out The MailboxesBecause ISPs nearly always provide dialup+email+pop together, most people are unaware that the three components can be separated. There is no requirement that the three services be bound together, though, for most ISP customers, it is convenient that they are. Yet they are entirely separate. Here are some examples of the ways in which the services can be separated. Directing other email addresses to a POP mailbox: With the right domain ownerships and authorities, you can configure other email addresses to point to your POP mailbox as well as the email address that your ISP has created for you. For example, if you registered your own domain daveadoodle.com, you could route the email address dave@daveadoodle.com to your POP mailbox. This can be done in Sendmail (by a system administrator) simply by mapping dave@daveadoodle.com to dave56@sooperdooperisp.com. There is no limit to the number of email addresses that you can point at a single POP mailbox. Having more than one POP mailbox: If you sign up at more than one ISP, you will have more than one POP mailbox. There is no limit to the number of POP mailboxes you can have. If you dialup the internet through one ISP, you can access your POP mailboxes at all the other ISPs too (unless they have put a special local-block on them). You don't (usually) need to dial up through an ISP to access your POP mailbox at that ISP. To retrieve your email from more than one POP mailbox, configure your email client to do so and your email client will then automatically retrieve your email from all your POP mailboxes whenever you "check your mail". Thus, if you wanted to, you could purchase a dialup
account from one ISP and use it to connect to a POP mailbox
hosted by a different ISP that receives email routed from
your own personal domain name hosted by a third ISP. Each
service is independent from the other services and can be
hosted on different ISPs.
Why Should I Care About POP Mailboxes?For most internet users, there is no need even to think about POP mailboxes. The users have a dialup account and an email address, and they can fetch their email and that's all that matters to them. That's fair enough. However, some basic knowledge about POP mailboxes becomes useful when you switch ISPs. For example, consider our hypothetical internet user Dave who owns his own domain name daveadoodle.com and who uses dave@daveadoodle.com as his official email address and who uses the ISP called Sooper Dooper ISP. We then have the following chain:
dave@daveadoodle.com --->
dave56@sooperdooperisp.com --->
Dave's POP mailbox at Sooper Dooper ISP
Suppose that Dave becomes dissatisified with Sooper Dooper ISP and wants to change ISP. Suppose he signs up with Grouse ISP. Then his new configuration will look like this:
dave@daveadoodle.com --->
dave12@grouseisp.com --->
Dave's POP mailbox at Grouse ISP
Simple enough, but consider the process of changing over. As Dave has his own domain name daveadoodle.com, and if has been disciplined enough to publish only his "official" email address dave@daveadoodle.com, then he won't need to inform any of his correspondents of the change of ISP. He will, however, have to take some care with POP mailboxes if he wants to ensure that no email is missed. The way to perform the changeover is as follows: 1. Sign up at the new ISP: The new ISP will provide Dave with a new POP mailbox and email address associated with it (dave12@grouseisp.com). 2. Redirect the official email address: The next step is to redirect the official email address dave@daveadoodle.com to the email address associated with the new POP mailbox (dave12@grouseisp.com). This is done by asking the ISP that is managing the domain name daveadoodle.com to modify its email routing settings. 3. Retrieve all email from the old POP mailbox: Now that we can be sure that no more email will arrive at the old POP mailbox, Dave should one last time retrieve all the email from the old POP mailbox. 4. Instruct the old ISP to delete the POP mailbox and email address: Now that the official email address points elsewhere, and now that Dave has performed one last check of the old POP mailbox, it is now safe for him to delete the old POP mailbox and old email address. If you don't perform something approximating this
procedure, some email might get lost (specifically email
that arrives after your last check of the old POP mailbox
and the time you redirect your official email address to the
new POP mailbox).
POP Mailbox Frequently Asked QuestionsThis section contains some specific questions and answers about POP mailboxes. Q: What is a POP mailbox? A POP mailbox is a place where email messages that have been received for you are stored before you get around to fetching them. Your POP mailbox stores each email message during the time when someone sends the email to you and the time when you "check your email". Q: How do I retrieve my email from a POP mailbox? Before you can retrieve email from a POP mailbox, you must first establish a connection to the internet (e.g. by "dialing up"). You must then invoke an email client such as Eudora or Outlook express and configure it to retrieve email from the POP mailbox from which you want to retrieve email. To configure it, you need to enter your POP mailbox's host, username and password. Q: Is my POP mailbox username and password the same as my dialup username and password? Probably. Some ISPs set the two to be the same. However, dialup services and POP mailbox services are entirely separate, so there is no technical requirement for your dialup username and password to be the same as your POP mailbox username and password. Q: What information do I need in order to be able to access a particular POP mailbox? To access a particular POP mailbox, you need (1) the name of the computer on which the POP mailbox resides (e.g. pop.sooperdooperisp.com), (2) the name of the POP mailbox (e.g. dave) and (3) the password for the POP mailbox. Q: What does POP stand for? POP stands for the "Post Office Protocol" which is the protocol that is run on the POP TCP port on computers running POP servers. The version of this protocol that is most widely used at the time of writing is V3, so the protocol is often known as the POP3 protocol. Q: What TCP port does POP run on? The POP3 protocol runs on TCP port 110. Version 2 of the protocol — POP2 — runs on TCP port 109. There is also a more secure protocol that uses TLS/SSL called POP3S which runs on TCP port 995. Q: Is a POP mailbox bound to a particular email address? Yes and no. From a technical point of view, a POP mailbox simply exists in its own right and there is no requirement to bind it to a particular email address or addresses. However, in practice, when an ISP creates a POP mailbox, the ISP nearly always binds the POP mailbox to a particular new email address in the ISP's email namespace (e.g. dave@sooperdooperisp.com). Let's call this email address the POP mailbox's "canonical address". Q: Can I direct more than one email address to a single POP mailbox? Yes. You can direct zero, one, two, three, or any number of email addresses to a particular POP mailbox. This is done by configuring the relevant domain name servers and email routers to route email to those addresses to the POP mailbox's canonical addresss (see above). Q: Can I have more than one POP mailbox? Yes, and they can be scattered all around the world. Q: Can I configure my email client to retrieve mail from more than one POP mailbox? Yes. Most email clients allow you to specify more than one POP mailbox and retrieve information from all the POP mailboxes whenever you "check your mail". Q: If I have a POP mailbox on the other side of the world, will it cost more to check my email? No. Most ISPs charge by the hour or by the megabyte, so it doesn't matter where your POP mailbox is. Q: If I have a POP mailbox on the other side of the world, will it take longer to check my email? Probably not. The main global backbone of the internet is very strong now and it is unlikely that the extra distance will introduce any significant delay. Q: Can I direct a particular email address to more than one POP mailbox? Yes. This can be done by creating a small mailing list in a Sendmail configuration that sends email directed to a particular address to the canonical email addresses of the various target POP mailboxes. Q: Does my POP mailbox have to be hosted by the ISP through which I dialup? No. Your POP mailbox is a completely separate service and can be hosted elsewhere. Q: When I send email, does it get sent through a POP mailbox? No. When you send email, your email client connects to an entirely different mail service called SMTP. SMTP servers accept email messages and deliver them to the correct POP mailboxes around the world. It is possible to configure a POP server to accept outbound email and this facility can work very well, but it is unusual. Q: How secure is POP? Not very. When you connect to your POP mailbox's POP server, your email client sends your POP username and password through the internet as plaintext (i.e. unencrypted). This means that, if anyone is listening, and they capture your username and password, they will be able to access your email. Q: If someone has my POP mailbox server name, mailbox name and password, can they read my email without my noticing? Yes. If someone has all this information, it is possible for them to connect to your POP server and retrieve a copy of the messages stored on the server at the time they connect, without deleting the messages from the server. This means that it is important to change your POP server password if it is ever accidentally disclosed. Q: What's APOP? APOP stands for Authenticated POP. It's a special feature of the POP protocol that allows you to connect to a POP server without disclosing your username and password to anyone who is listening to the network conversation. If you connect to a POP server using the APOP protocol, then your username and password is not transmitted in plaintext during the authentication phase of the POP conversation. However, your email is still tranferred unencrypted. Thus, APOP prevents someone who is listening in from accessing your POP mailbox, but it does still allow them to listen to your email as it flies past. Q: What's IMAP? IMAP is a totally different
email retrieval system/protocol. It is entirely independent
and separate from POP, but can be used as an alternative to
POP. This web page does not cover IMAP.
GlossaryCheck Your Mail This phrase usually means that you invoke your Email Client and instruct it to retrieve all your email messages from one or more POP mailboxes. Email Client Software that enables you to retrieve email from a POP mailbox and store and manipulate the email. Examples of email clients are: Eudora, Outlook, and Outlook Express. ISP An Internet Service Provider. POP Server An internet server (a la "web server") that accepts connections from Email Clients and (subject to receiving a correct username and password) sends the client the messages stored in a particular POP mailbox. A single POP Server will usually "serve" many different POP mailboxes. Ross Williams (ross@ross.net)
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