SMTP PROTOCOL CONFIGURATION AND MANAGEMENT

Chapter 8

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OVERVIEW

SMTP and ESMTP DNS MX records Internet connectivity SMTP virtual servers and connectors Relaying and smart hosts SMTP security Global settings Domain nodes Per-user settings

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HOW SMTP IMPLEMENTS A CONNECTION

Initiates a TCP connection Response 220

HostHost ServerServer

Indicates end of session using quit

Sends helo command Response 250

Identifies the sender using mail from: Response 250

Identifies the recipient using rcpt to: Response 250

Indicates ready to send using data Response 354

Sends message Waits for quit

Response 221

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HOW ESMTP IMPLEMENTS A CONNECTION Host sends ehlo instead of helo If server supports ESMTP it returns

response 250 If server does not support ESMTP it

returns response 500 ESMTP session very similar to SMTP

session

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SMTP SYSTEM FOLDERS

Pickup Queue Badmail

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CONFIGURING MX RECORDS

Managing your own DNS: Single namespace Multiple namespaces

Internet service provider (ISP) manages your DNS: Nonpersistent connection Permanent connection

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CONFIGURING INTERNET CONNECTIVITY Configuring SMTP virtual servers Creating and configuring an SMTP

Connector Configuring an SMTP policy for a domain Configuring per-user settings

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DEMONSTRATION: CREATING AND CONFIGURING AN ADDITIONAL SMTP VIRTUAL SERVER

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DEMONSTRATION: CREATING AN SMTP CONNECTOR

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DEMONSTRATION: CONFIGURING AN SMTP CONNECTOR Limiting the scope Configuring the

credentials Configuring to only

receive e-mail Configuring to only send

e-mail Configuring Internet

message formats Configuring message

delivery parameters

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SMTP RELAYS

SMTP virtual server configured to use a smart host

SMTP virtual server forwards unresolved messages to a smart host

SMTP virtual server configured as a relay host

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SMTP RELAYS (CONT.)

SMTP virtual server configured to limit the servers that can relay e-mail messages

SMTP Connector configured to use a smart host

Configuring domains to which you want to relay messages

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SMTP VIRTUAL SERVER CONFIGURED TO USE A SMART HOST Virtual servers forward all outbound mail

to a smart host Virtual server does not resolve the SMTP

domain name Entry and exit point for all Internet

messages Entry and exit point for messages to a

foreign messaging system

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SMTP VIRTUAL SERVER CONFIGURED TO USE A SMART HOST (CONT.) Helps manage Internet message traffic Provides dial-up solutions Clients do not need permanent

connections to the Exchange server

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SMTP VIRTUAL SERVER FORWARDS UNRESOLVED MESSAGES TO A SMART HOST Forward all unresolved SMTP messages

from Exchange to a smart host Other SMTP messaging systems in

addition to Exchange Smart host cannot resolve the

recipient’s name; message returned with a nondelivery report (NDR)

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SMTP VIRTUAL SERVER CONFIGURED AS A RELAY HOST Configure an SMTP virtual server as an

inbound relay host Gives Exchange Server 2003 smart host

capabilities Can configure other SMTP servers to use

the virtual server as their smart host Virtual server resolves the recipient’s

SMTP domain name through DNS and delivers the messages

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LIMIT THE SERVERS THAT CAN RELAY E-MAIL MESSAGES Specify who or what can relay e-mail

messages through your organization: Computers Groups of computers Domains

Prevent unwanted SMTP hosts from using your SMTP host as a relay agent

Stops third parties from relaying bulk unsolicited commercial e-mail

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SMTP CONNECTOR CONFIGURED TO USE A SMART HOST By default SMTP Connector uses DNS Can configure connector to forward all

outbound mail to a smart host Typically SMTP configuration done on

connector rather than virtual server

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CONFIGURING DOMAINS TO WHICH YOU WANT TO RELAY MESSAGES Can limit domains to which you relay

messages Useful when organization has multiple

SMTP messaging systems and domain names

SMTP host can accept messages from any domain but then forward them only to specific domains

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VERIFYING A CONNECTION BETWEEN AN SMTP CONNECTOR AND A SMART HOST Send an e-mail message to an

unresolvable address on the smart host Verify the connection object in the

queue

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CONFIGURING CONNECTIONS ON AN SMTP VIRTUAL SERVER Incoming:

Limit Number Of Connections To Connection Time-Out (Minutes)

Outgoing: Limit Connections To Time-Out (Minutes) Limit Connections Per Domain To TCP Port

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SMTP SECURITY

Authentication Encryption Reverse DNS lookup

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AUTHENTICATION

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ENCRYPTION

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REVERSE DNS LOOKUP

IP spoofing: Attacker impersonates a trusted host Uses its IP address

Reverse DNS lookup: Resolves IP address to a host name or FQDN Confirms that the sender’s IP address is

from the correct network Result written into the message’s SMTP

header

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RESTRICTING INTERNET E-MAIL

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RELAYING

Permits mail for another organization to be forwarded

Disabled by default Required for interfacing with other SMTP

mail systems Required to allow IMAP4 and POP3

clients to send mail

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OPEN RELAYING

Organization configured to allow relaying by default

Open relaying allows propagation of junk mail

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CONFIGURING RELAYING

Can restrict using discretionary access control lists (DACLs)

Safer to create additional SMTP virtual server

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RETRIEVING E-MAIL FROM AN ISP Typically over a nonpersistent connection Configure the on-demand dial-up connection in

Routing and Remote Access Service Configure ISP’s Exchange server as smart host Pull e-mail by using the turn or etrn command Advanced tab of SMTP Connector’s Properties

dialog box Request ETRN/TURN when sending messages

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MESSAGE DELIVERY FAILURES

Identify where failure occurred SMTP host unable to deliver:

Test using telnet DNS problem:

Test using nslookup

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OTHER SMTP MESSAGING SYSTEMS

If connectors do not exist: Obtain third-party gateways Use Microsoft Exchange 5.5 connectors

Microsoft Mail: Configure Exchange 2000 Server for

directory synchronization

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GLOBAL SETTINGS

Configure systemwide settings Overridden by:

Virtual server settings Per-user settings

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SMTP POLICY ON A DOMAIN NODE

Does not create a new domain Used when sending messages in a

format suitable for another domain Can be used to send mail to a partner

organization Can be used for interdomain mail within

the same forest

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CONFIGURING AN SMTP POLICY

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MAILBOX DEFAULTS

Mailbox defaults apply to all mailboxes Per-user settings apply to individual

mailboxes Widely used to prevent bottlenecks in

the Exchange routing engine Message size limits can apply to

inbound or outbound messages Recipient limits apply to all messages

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PER-USER SETTINGS FOR OUTLOOK WEB ACCESS

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PER-USER SETTINGS FOR IMAP4 AND POP3

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SUMMARY

How SMTP and ESMTP work Identifying Exchange servers and

connecting to the Internet SMTP virtual servers, connectors, relays,

and smart hosts Security: authentication, encryption,

reverse DNS lookup Global settings and SMTP policies Per-user settings