How to block users from sending mail in linux

There are several ways to block users from sending mail in Linux, depending on your specific setup and requirements. Here are a few methods:

Method 1: Using Postfix

If you're using Postfix as your mail server, you can block users from sending mail by modifying the main.cf file. Add the following lines to the file:

smtpd_recipient_restrictions = reject_unauth_destination
smtpd_sender_restrictions = reject_sender_login_mismatch

This will reject any mail that doesn't have a valid sender address and doesn't match the login credentials of the user sending the mail.

Method 2: Using Sendmail

If you're using Sendmail as your mail server, you can block users from sending mail by modifying the sendmail.cf file. Add the following lines to the file:

Dj reject_unauth_destination
Dj reject_sender_login_mismatch

This will reject any mail that doesn't have a valid sender address and doesn't match the login credentials of the user sending the mail.

Method 3: Using Exim

If you're using Exim as your mail server, you can block users from sending mail by modifying the exim.conf file. Add the following lines to the file:

acl_smtp_rcpt = reject_unauth_destination
acl_smtp_sender = reject_sender_login_mismatch

This will reject any mail that doesn't have a valid sender address and doesn't match the login credentials of the user sending the mail.

Method 4: Using a firewall

You can also block users from sending mail by configuring your firewall to block outgoing mail traffic on port 25 (the default SMTP port). For example, if you're using ufw (Uncomplicated Firewall) on Ubuntu-based systems, you can run the following command:

ufw deny out 25

This will block all outgoing mail traffic on port 25.

Method 5: Using a mail filter

You can also block users from sending mail by using a mail filter like SpamAssassin or ClamAV. These filters can scan incoming and outgoing mail for spam and viruses, and block or reject mail that doesn't meet certain criteria.

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