Page 153 - StudyBook.pdf

Communication Security: Remote Access and Messaging • Chapter 3 137

E-mail servers use a number of different servers to locate the IP address of a
recipient’s domain.An IP address is a unique number that identifies computers on
the Internet, to ensure messages get to the correct destination. Because
mybuddy@receivingcompany.com is a .com domain, the ISP’s server contacts a Domain
Name System (DNS) server authorized to find IP addresses of name servers in the
.com domain.The e-mail server then sends out several requests to the name servers
to find the IP address of the recipient’s domain (receivingcompany.com).The DNS
server looks up the mail exchange (MX) record for receivingcompany.com, and
responds to the e-mail server with the IP address.After the e-mail server has the IP
address, it sends the message.When the e-mail server at receivingdomain.com gets the
e-mail message, it will be placed in the recipient’s mailbox, based on the mailbox
account name.

Figure 3.13 How E-mail Gets from Sender to Recipient

DNS server

2. Mail server queries
Internet DNS 3. DNS returns MX record for target
for MX record of domain, mail.receivindomain.com,
receivingdomain .com IP 217.123.456.789
Email
client server Internet
Email
server
1. Message is sent from
mail client to mail server
Sendingcompany.com 4. Message is sent to the IP address
Specified in the MX record for Receivingcompany.com
receiving domain

To make the e-mail process clearer, think of e-mailing in terms of sending
physical interoffice mail from one department to another. Mail is given to a
delivery person who looks at the envelope’s destination. From the address, the
delivery person knows that the recipient is on the third floor.The delivery person
goes to the third floor and asks someone where the specific department is.When

www.syngress.com