Q. How do I block some1@domain.com or IP address for Sendmail spammers?

A. Sendmail offers various options to block an email address, domain name or IP address using access database.

File /etc/mail/access act as an access control for your Sendmail smtp server. With this file you can define:

1. Who we accept mail from
2. Who we accept relaying from
3. Who we will not send to etc

Configuration

# vi /etc/mail/sendmail.cf

Make sure following line exists in your configuration file (otherwise you need to add/append following line):

FEATURE(`access_db’)dnl

Save the file and exit to shell prompt. Now, open /etc/mail/access file:

# vi /etc/mail/access

To block junk or spam mail use following format
spam@domain.com REJECT
192.168.1.100 REJECT
202.54.1.100/255.0.0.0 REJECT

Save the file. Once sendmail.mc or access file is modified, you need to regenerate /etc/mail/sendmail.cf or rebuild the access database. Type any one of the following commands:

# cd /etc/mail
# make

OR

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Restart the sendmail:

# /etc/init.d/sendmail restart

Other options supported by access database

• OK – Accept email FROM: or TO: specified address
• DISCARD – Do not accept email FROM: or TO: specified address
• REJECT – Drop email with an error FROM: or TO: specified address
• HATER – Use check_mail and check_realy rules for email
• FRIEND – Do not use check_mail and check_realy rules for email
• ERROR:ecode:etext – Drop email with specified error code (ecode) and error text message (etext).

Examples

abc.com ERROR:550:Relay denied
FROM:mail@fack.net REJECT
TO:foo@bar.com REJECT
Spam:abuse@ FRIEND
Connect:202.54.1.1 ERROR:5.7.1:550: Relay denied

How do I test access database is working or not?
Use sendmail with -bt option as follows:

# sendmail –bt
> check_mail user@abc.com

OR use telnet command:

# telnet localhost 25
HELO localhost
MAIL FROM:user@abc.com

In both case you should get relay denied error.

See also:

• Test mail server for an open relay

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