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The Server is Being Used to Send SPAM

Make sure that the BM server isn't an open relay​

By default, Postfix is configured as an open mail server relay, which allows BlueMind services (webmail, EAS) to send emails. Through the admin console, you can add new trusted IPs which will be allowed to send emails through the BlueMind server, typically the internal IT server (holiday management software, monitoring, etc).

If one of these servers is compromised, a spammer can use the BM server to send spam.

If you have this issue, you'll find a large number of command lines such as:

Jun 21 08:34:30 centos7 postfix/smtpd[16863]: connect from gateway[192.168.122.111]
Jun 21 08:34:48 centos7 postfix/smtpd[16863]: 1C205316C411: client=gateway[192.168.122.111]
Jun 21 08:34:58 centos7 postfix/cleanup[16869]: 1C205316C411: message-id=<>
Jun 21 08:34:58 centos7 postfix/qmgr[16801]: 1C205316C411: from=<user1@domain.com>, size=233, nrcpt=1 (queue active)
Jun 21 08:34:59 centos7 postfix/smtp[16821]: 1C205316C411: to=<user2@anotherdomain.net>, relay=smtp.anotherdomain.net[XX.XX.XX.XX]:25, delay=20, delays=20/0/0.35/0.09, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 2F10238)
Jun 21 08:34:59 centos7 postfix/qmgr[16801]: 1C205316C411: removed
Jun 21 08:35:04 centos7 postfix/smtpd[16863]: disconnect from gateway[192.168.122.111]

The client line indicates the IP of the server that sent this e-mail, here 192.168.122.111. If it's not legitimate, this means that the server has been compromised and is being used to send spam

Take the following action:

  • delete the IP for the server involved in the BM admin console and save the configuration.

Identifying the user used to send SPAM​

Most of the time, the issue is that a spammer has found – often using brute-force – a user's password and uses their account to send SPAM.

You will spot smtp connections with many command lines such as:

May  5 00:08:55 hermes postfix/smtpd[27666]: 7E079B666CC: client=unknown[46.48.93.60], sasl_method=LOGIN, sasl_username=admin

Take the following actions:

  • change the user's password: either in BM, or in the directory.

  • close all sessions for this user with the command :

    bm-cli user logout --email=login@domain.com

  • clean the Postfix queue to delete emails pending to be sent for the user "login@domain.net".

    postqueue -p | tail -n +2 | awk 'BEGIN { RS = "" } /login@domain\.net/ { print $1 }' | tr -d '\*!' | postsuper -d -

    ❗ This command will delete all mail in the user's queue, whether SPAM or legitimate mail sent by the user.

Preventing spam attacks​

Limiting brute-force attacks​

To try and avoid brute-force attacks, you can use the plugin password-bruteforce which blocks connection attempts for 20 minutes after 3 failed authentications.

Set up a password policy​

In case you are not using a directory to manage users, you can use the plugin password-sizestrength which can be used to establish a policy to enforce rules for passwords.

How to spot this early?​

A monitoring solution can help detect the issue quickly and react before your server becomes blacklisted by anti-spam services.

Bm-tick can be used to set up alerts when a strong increase in the number of emails is detected in the Postfix queue. You can configure this using the alert builder or the following script: postfix_queue.tick

This script sends an alert email to admin@domain.net as soon as the Postfix queue reaches 200 messages. These values can be configured to match your installation.

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