Otras teorías sobre la función ejecutiva

1. mail to: [email protected]

SMTP MTA (client)

IN MX 10 mx1.acme.com IN MX 20 mx2.acme.com IN MX 30 mx3.acme.com IN A 192.168.10.20

SMTP Integration with the DNS

This model is popular with consumer services and free Internet access providers, but is not widely used by business users because it seems less professional than a dedicated domain. In this model, users are simply given a user account at the ISP domain, of the form:

[email protected]

Because the username chosen must be unique across a potentially very large domain, users are normally forced to use digits as well as common names.

E-mail is sent using SMTP, and retrieved using POP3 or IMAPv4. Both schemes allow for user authentication using a simple shared secret password. Mail is sent and received between servers using SMTP, and normally a secondary and tertiary mail server will be configured within the ISPs own domain to provide resilience and disaster recovery.

In the example shown, the ISP operates a separate mail server for inbound and outbound traffic. The inbound SMTP server also provides a POP3/IMAP4 server for access to e-mail by the dial-up accounts. The outbound SMTP server need not have an MX record in the DNS, as it only ever originates transfers to the Internet. A secondary MX server is also available should the primary be unreachable from the Internet.

To provide an even more scalable solution, it is possible to separate the mail handling function into three parts:

• an inbound SMTP server, which has an MX record in the DNS. It accepts all domain e-mail, and relays it to a separate POP3/IMAP server

• a POP3/IMAP server, which provides the mailboxes and access for users. This server only accepts mail from the ISP’s own inbound SMTP server

• an SMTP outbound server, which accepts e-mail from users, and sends it out to the Internet

This architecture should have one or more secondary MX machines, as before, which can accept inbound SMTP e-mail if the primary is unavailable for any reason.

Outbound e-mail to Inte(SMTP)rnet

(SMTP)

IN MX 10 mx1.myISP.com IN MX 20 mx2.myISP.com myISP.com

Figure 13

Simple Service Provider Mail Accounts

Many small to medium-sized businesses require Internet e-mail for a set of internal users, but cannot easily run an enterprise mail server internally. This may be because they use dial-up access and have no permanent IP address, or because their use of e-mail internal to the organization does not justify a corporate e-mail infrastructure.

In this case, many service providers offer Internet e-mail services linked to a dedicated customer domain, and often bundled with web hosting services.

In this case, the ISP will typically organize domain registration through an ICANN approved registrar, if the domain has not yet been registered. If the domain has been registered, but the customer is transferring to a new service provider, then the registration details must be modified to identify the correct name servers for the domain, normally the name servers of the new ISP.

In this scenario, mail between users within the customer domain is sent via the domain mail server within the ISP network, just as for external mail. In other words, all e-mail is Internet e-mail.

The mail delivery methods are the same as for the earlier example, typically SMTP for outbound mail, and POP3 or IMAP4 for inbound delivery. All corporate users must have the correct details for their account configured in their mail client, including server names, username and password.

As the customer now has a registered domain, the DNS records for this domain will show that mail is being handled by the ISP mail servers, as shown in Figure 14.

The ISP will not actually run a separate instance of a mail server, physically or logically, in this case. Instead the e-mail addresses of the customer may be mapped by a mail alias onto an internal account on the ISP server. So, for example [email protected] might be mapped to [email protected]. This retains the uniqueness of the user name, but maps it into the ISP mail namespace.

The mail alias is then used to translate inbound and outbound mail transfers between the two names. An alternative approach is to provide a virtual mail server for each domain, and many mail server applications allow this.

Outbound e-mail (SMTP)

Inbound e-mail (POP

3 or IMAP4)

from Internet to Inte(SMTP)rnet

(SMTP)

IN MX 10 mx1.myISP.com IN MX 20 mx2.myISP.com wraycastle.com

Outsourced Customer Mail Service

Where a business operates an existing Internet mail server, they may require that a service provider offer a secondary mail server, to improve the resilience of the service. These resilience measures are particularly important once key servers, including mail, DNS and web servers, are hosted at customer premises; the reliability of the access circuits, power circuit and equipment at these locations is normally much poorer than that to the data centres where network servers are typically located.

A common architecture is to configure a secondary mail server within the ISP domain as a mail relay. In this model, the relay will attempt to deliver mail on to the primary server (at the customer premises) for some period of time, before eventually deleting the mail, perhaps after several days.

This approach can also be used where a dial-up network has a legitimate domain and a primary mail server with an MX record in the DNS. In that case, the secondary mail server effectively acts as a store and forward server for the corporate e-mail when the dial-up connection is down, then relays it to the primary mail server when the dial-up connection comes up.

Mail delivered

IN MX 10 mx1.wraycastle.com IN MX 20 mx2.myISP.com wraycastle.com

ISP Secondary Mail Servers