THE VIRTUAL SERVER HANDBOOK

POP(IMAP) before SMTP Relay

Blocking

Unauthorized SMTP relaying is a growing abuse trend, usually used by

individuals or groups of individuals to send large amounts of unsolicited E 

mail, typically of a commercial nature.

An SMTP relay incident occurs when an SMTP server is requested to

deliver an E mail message that is not destined to any of its local users. The

SMTP server passes the message on to another SMTP server, hence the

term "relay", which in turn routes it to the eventual recipient user. SMTP

relaying enables the injection of legitimate E mail messages into the mail

system from client machines that do not offer full SMTP server capabilities

such as many PCs running Windows or Macintosh computers.

However, it is a growing trend for "Spammers" to locate unprotected or

"open" SMTP servers that can be used as SMTP relays for unsolicited E 

mail campaigns. Unscrupulous individuals hijack your SMTP server,

sending your SMTP server a single copy of a message, then requesting that

your SMTP server relay the message to recipients. Many servers crash in

the aftermath under the sheer load of bouncing E mail or complaints from

spam recipients.

In the default configuration, the virtual server's SMTP server is closed to all

users unless they have a valid username and password. This shuts down

relaying and protects the virtual servers resources. To do this, the virtual

server system uses a technique sometimes called "POP before SMTP"

(since it also applies to the IMAP server, it could also be called IMAP 

before SMTP) to limit SMTP relaying to users who have previously

accessed the POP server (or the IMAP server) with their password.

POP before SMTP relay blocking works every time someone successfully

enters a correct username and password to the POP server. The POP server

records the remote client IP address for later use by the SMTP server.

Because of POP before SMTP relay blocking, your users must check their

E mail (by accessing either the POP server or the IMAP server) before they

try to send E mail. The SMTP server refuses to accept their outgoing mail

message otherwise.

Note:  POP before SMTP relay blocking has the largest effect on users

who are dynamically allocated an IP address each time they connect to

the Internet.

To configure your E mail clients to authenticate before sending

mail

1.

From "check mail every x minutes" set the number of minutes to any

number. The check mail option makes the E mail client authenticate

first before sending.

2.

Newer E mail client software has POP before SMTP setup options.

Choose the "authenticate before sending" option.

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 1999  DIGITAL TOOLS  LLC.

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