A basic understanding of how email works is critical
to managing an email system efficiently. Email is a client/server
application that enables a client to send messages to any other client
with only a simple identifying address. Between the sending and
receiving clients is a system of email servers that communicate with
each other using specialized protocols, such as the Simple Mail Transfer
Protocol (SMTP).
As with most networking subjects, email communication can be extremely
complicated, but the typical small business network administrator does
not need to delve into the technical details too deeply. The following
sections examine some of the most basic concepts, however, and describe
how they pertain to Windows SBS 2011.
1. Understanding Email Addresses
As all email users know, an email address consists of a single user name, followed by an @ character and a domain name, as shown in Figure 1. The first part of the address, the part before the @ sign, is the local part, which needs to be understood only by the destination mail server. The part after the @ sign identifies the domain on the Internet where the destination client is located.
Routers on the Internet use
the network identifier to forward IP datagrams to a particular
destination network and then the router on the destination network uses
the host identifier to forward the datagrams to the correct computer on
that network. In the same way, the Domain Name System (DNS) identifies
computers using fully qualified domain names (FQDNs), which consist of
two parts: a host name and a domain name. When a DNS server tries to
resolve an FQDN into an IP address, it forwards the name resolution
request to the authoritative server for the domain, which looks up the
IP address of the specified host.
Email communications function in much the same way. The SMTP
servers on the Internet read only the second part of the email address
and forward the email message to the mail server for the appropriate
domain. Then, the domain mail server reads the first part of the
address—the part before the @ sign—and deposits the email message in the
mailbox for the appropriate user.
Because the domain name of an
email address must be understandable to all the servers on the Internet,
it must conform to the same standards as all DNS domain names.
Therefore, the domain name part of an email address is subject to the
following limitations:
The domain name can be no more than 255 characters long.
Domain names can consist only of the letters A to Z, the numbers 0 to 9, and the hyphen (-) character.
Domain names are not case-sensitive.
Because the local part of an
email address has to be read and understood only by the destination mail
server, its specifications are less stringent. The local part of an
email address is subject to the following limitations:
The local part of the name can be no more than 64 characters long.
Local part names can consist of the letters A to Z, the numbers 0 to 9, and the following characters: ! # $ % & ´ * + - / = ? ^ _ ` { | } ~.
Local
part names can also contain the period (.) character as long as it does
not appear as the first or last character and as long as it does not
appear twice in succession.
Local part names can conceivably be case-sensitive, but in Exchange Server 2010, they are not. Exchange Server delivers the addresses sanjays@adatum.comSanjayS@adatum.com to the same mailbox. and
Local part names can be
case-sensitive because their interpretation is left solely to the
destination email server. If a particular server implementation supports
case-sensitive local part names, and the destination server is running
that implementation, then the distinction of two local part names that
differ only in their case is possible. However, on the Internet, senders rarely know what server implementations their recipients are using, so most email
servers, including Exchange Server 2010, follow the recommendation of
the SMTP standard and treat all local part names as case-insensitive.
Windows SBS 2011 does not allow you to create two user accounts with email addresses that differ only in case.
Note:
Some email servers impose
other restrictions on local part name construction. For example, the
Windows Live Hotmail system limits local part names to letters; numbers;
and the period (.), hyphen (-), and underscore (_) characters. You
cannot create a Hotmail account name using any other characters, and the
Hotmail system does not send email to any address using other
characters.
Despite the limitations
listed earlier, one of your primary goals when assigning email addresses
should always be user-friendliness. An email address like hknjv!fgjyc8*pi09iponi0-v665q{436y@xyucu6ysxxgfu7opm83opdx5zw56iyb.com
would be technically legal, but it would be terribly inconvenient for
the individuals forced to use it or anyone trying to remember it.
2. Understanding Email Server Functions
Email clients have two
basic messaging functions: They send outgoing mail to one kind of server
and they retrieve incoming mail from another. The servers conduct the
rest of the email communication process, including the transmission of
messages to computers hundreds or thousands of miles away. The following
sections discuss the main email server types.
Note:
It is critical to realize that in this discussion of email communications, the term server
does not necessarily refer to a separate computer, but instead to a
process running on a computer in the form of an application or service. A
single computer can perform multiple server functions, as in the case
of a computer running Exchange Server 2010, which can perform all the
email server roles simultaneously.
2.1. Simple Mail Transfer Protocol (SMTP)
SMTP is the primary email communication protocol, responsible for the majority of email traffic on the Internet. Every email
client has the name or IP address of an SMTP server in its
configuration settings, to which it transmits its outgoing mail
messages. Email servers can use SMTP for both incoming and outgoing traffic.
SMTP is a text-based,
application layer protocol that email clients use to send their outgoing
messages to a server, and email servers use it to forward the messages
to other servers. Windows SBS 2011 servers function as SMTP
servers, as can all computers running Exchange Server 2010. Whichever
email client your users choose to run, that client sends its outgoing
email messages to the Windows SBS 2011 server using SMTP. If the
intended recipient of a message is another user on your network, the
Windows SBS server deposits the message in the recipient’s Exchange
mailbox. If the message is addressed to a user in another domain, the
server transmits the message to another SMTP server on the Internet.
An SMTP server is a
relatively simple mechanism, but its role has been complicated over the
years by the increasing prevalence on the Internet of unsolicited email
traffic, also known as spam.
In earlier days, Internet service providers (ISPs) set up SMTP servers
for their customers, connected them to the Internet, and left them open
for use by anyone. The well-known port number for the SMTP protocol is
25, and those servers willingly accepted anyone’s outgoing SMTP email
messages as long as they were addressed to that port.
However, it was not long before
spammers began using these open servers to send millions of unsolicited
messages. By using the SMTP servers belonging to other ISPs, the
spammers made it difficult, if not impossible, to trace their spam
emails back to them. As a result of the enormous amounts of bandwidth
consumed by the spam, ISPs were forced to add various forms of
protection to their SMTP servers.
Most Internet SMTP
servers today require users to authenticate before they can submit
outgoing traffic, and many of them refuse all traffic addressed to port
number 25. Email clients typically enable users to specify the
credentials they should use to log on to the SMTP server, as shown in Figure 2,
as well as an alternative to port number 25. Port number 587 has been
standardized as the port for authenticated outgoing mail submissions,
but some ISPs use nonstandard ports instead.
Note:
On a Windows SBS 2011 network, the computer functioning as the SMTP
server is not accessible directly from the Internet, so it is not
subject to abuse by spammers outside the local network. Therefore, it is
not necessary to take these protective measures.
2.2. Post Office Protocol Version 3 (POP3)
SMTP is strictly a “push” protocol. Email clients and other email
servers send messages to SMTP servers; they do not retrieve messages
from them. To retrieve their incoming messages from a server, clients
use one of two “pull” protocols: Post Office Protocol version 3 (POP3) or Internet Message Access Protocol version 4 (IMAP4). POP3 is the more popular of these protocols.
Note:
The standard for version 3
of POP was published in 1996. POP1 and POP2 have long since become
obsolete, and any reference to POP without a version identifier almost
certainly refers to POP version 3. There is a Post Office Protocol
version 4 (POP4) server in development, but the protocol has not yet
been standardized, nor is it commercially available.
POP3 is a relatively simple protocol that is designed to provide clients with offline access to their email
messages. A POP3 server maintains a separate mailbox for each user in a
particular domain, whereas the server stores the incoming email
messages it receives through its SMTP connections. Email clients
periodically connect to the server, authenticate the user, and download
the messages in the user’s mailbox. In most cases, the server deletes
the messages once the client has downloaded them, but many POP3
implementations provide users with the ability to leave copies of the
downloaded messages on the server, as shown in Figure 3.
The design of the
POP3 mechanism enables clients to connect to the server, download
messages, and then disconnect, after which the user can work with the
messages offline. Because of this, the client’s message store is said to
be authoritative in a POP3 application. When dial-up connections were
the prevalent form of Internet access, POP3 provided the most
bandwidth-efficient method of accessing incoming email.
POP3 is designed to
keep the server side of the application as simple as possible, leaving
the majority of the messaging tasks to the client. There are, however,
two potential areas of server complexity. One involves the numbering of
the messages in a mailbox when a user downloads and deletes some, but
not all of the waiting messages. Instead of numbering the messages
consecutively, and renumbering the messages when the client deletes some
of them, most POP3 implementations use a technique called Unique Identification Listing (UIDL) to assign a permanent, unique identifier to each message in the mailbox.
The other potential problem is one of authentication security. The POP3
standard contains no provision for the use of encrypted passwords, and
some implementations still require clients to transmit passwords in
plain text. There are, however, a number of POP3 implementations that
use security extensions to protect passwords and prevent unauthorized
access to email accounts.
POP3 servers use the well-known port number 110 for client connections, and many implementations can use Secure Sockets Layer (SSL) or Transport Layer Security (TLS) to encrypt the contents of the email messages during download.
2.3. Internet Message Access Protocol 4 (IMAP4)
IMAP4 is another “pull”
protocol that clients can use to obtain their email messages from a
server. However, unlike POP3, IMAP4 is designed to leave the messages
stored on the server and enable users to work with them there. An IMAP4
client is able to store copies of email messages on the local drive, but
the authoritative message store resides on the server.
Most email clients can
support both IMAP4 and POP3 connections to a server. IMAP4 connections
use well-known port number 143. ISPs tend to provide their customers
with POP3 mailboxes because they require fewer server resources and much
less server storage. Web–based email implementations, on the other
hand, often use IMAP4 to display a user’s message store in a web browser
interface.
IMAP4 places a much greater
burden on the server than POP3, not only because the server must
maintain a message store for each user but also because the IMAP4 server
provides more functions than a POP3 server. IMAP4 clients can create
folders to organize email messages, move messages around between
folders, and run searches for specific messages. Searching, in
particular, can be a highly resource-intensive task, depending on the
size of the mailbox.
IMAP4 also provides
distinct advantages for the user. When a client connects to a server
using IMAP4, access to the user’s message store is almost immediate
because the client is displaying the contents of the mailbox as it
exists on the server. By contrast, a POP3 client must check the server
for new messages, download them, and integrate the messages into the
client’s data store before the user can begin working with them.
Because IMAP4 stores
messages on the server, users can access their mailboxes from different
locations without causing problems. For this reason, IMAP4 is a popular
solution on college campuses, in which students in a computer center
might use a different system each time they access their email. IMAP4
also enables multiple users to access the same mailbox simultaneously,
while a POP3 mailbox can support only one connected user at a time. This
can be highly useful in a business environment, such as a help desk
that has several people servicing a single email help line.
2.4. Exchange Server 2010 Functions
Exchange Server 2010, although
based on industry standards, is a proprietary mail and scheduling
product that is designed to provide clients with access to local and
Internet email, shared
calendars and scheduling, task management, and a unified messaging
interface that can route other types of traffic, such as voice mail and
faxes, to a user’s inbox. Windows SBS 2011 automatically installs
Exchange Server 2010 with the Windows Server 2008 R2 operating system
and configures it to provide these services to your network users.
When you run the Add A New
User Account Wizard in the Windows SBS Console, the wizard creates an
Exchange Server mailbox for each of your new users using the email address you specify. By default, the email
address consists of the user’s account name and the name of the
Internet domain you specified in the Internet Address Management Wizard,
as in the example marklee@adatum.info.
Users can access their mailboxes using the Office Outlook Web Access (OWA) site, shown in Figure 4,
which Windows SBS 2011 creates by default. Users can also access their
Exchange Server mailboxes with Microsoft Outlook, but this client is not
included with Windows SBS 2011. You must purchase an appropriate
edition of Microsoft Office 2010 for your client computers to obtain the
Outlook client.
The Exchange Server 2010
implementation in Windows SBS 2011 includes POP3 and IMAP4 servers among
its capabilities, but by default, the server does not start the
Exchange POP3 and Exchange IMAP4 services, which prevents clients from
using these protocols to access their Exchange Server mailboxes. If
desired, you can start the POP3 or IMAP4 service on your Windows SBS
2011 server, enabling users to access their mailboxes using clients such
as Windows Live Mail, the Windows Mail client included in Windows
Vista, and the Outlook Express client in Windows XP. However, this
solution provides users with email access only. These clients do not support the scheduling and task management features in Exchange Server.
