To secure Postfix, there are a number of functions you can add the configuration file /etc/mail/main.cf. Edit the file and add the following lines:

smtpd_helo_required = yes
smtpd_delay_reject = no
disable_vrfy_command = yes

smtpd_helo_restrictions = permit_mynetworks,
reject_invalid_hostname,
reject_unknown_hostname,
reject_non_fqdn_hostname

This will force incoming requests to pause and identify themselves before proceeding, and will allow properly authenticated users to pass through and send mail. To finish, save the file and restart Postfix:

/etc/init.d/postfix restart

Answer: Usually, you will only be accused of spamming in one of three situations: (1) you actually are a spammer, (2) your account is hosted on an unsecured mail server, or (3) You have a dedicated server or VPS, and you have left your mail server unsecured.

In the case of the first situation, I cannot help you. If you are in situation number 2, then you are truly one of the victims, and whoever is running the server needs to fix the problem. Usually this will happen if the server admin is someone different than the web host. Contact the web host and let them know that you are not in charge of the server but have been blacklisted.

In the third situation, you need to find the security hole in your server and fix it. You could have an open relay in your SMTP system. Another possibility is that a user account has been comprised. And the third unfortunate possibility is that one of your users may knowingly be a spammer. In any case, get the problem fixed first and then tell your web host. You should then automatically start dropping off of DNS blacklists, and everything will go back to normal.

Image: Wikimedia Commons

Answer: DNSBL stands for DNS Block List or Blacklist. Essentially, it is a published list of ipp addresses that are in some way connected to spam. Although the most common form of DNSBL is a list of actually servers and/or computers that actually send spam, there are some that flag ip addresses based on association with spamming computers or the potential to send spam. The latter two are particularly controversial.

The reason you should care about DNSBL is that there is always the possibility that your server could end up on one. If you run a dedicated server with multiple users, the potential is greater because you could actually have users who abuse their privileges and send spam. As a result, their ip address will be flagged, and if that ip address is shared with others, everyone suffers.

The result is that any other email servers that rely on the lists will reject email sent from your server. You can prevent this by ensuring that your server is locked down and not an open relay for spam. Furthermore, you can periodically check the most common block lists to see if your ip addresses are listed. If they are, find out how spam is being sent through your server and eliminate the problem. Within 48 hours of fixing the problem, most lists will remove your IP addresses.

Photo: Flickr