Under the Mercury SMTP Server option, there is also a tab for Spam control. This is for adding Blacklist or Whitelist databases using DNS querying the IP addresses of any incoming SMTP connection and then refusing or allowing them based on the results. This is particularly useful for blacklisting spammer ip address blocks.
To add a blacklist DNS server, select Add, then enter a name for the definition (eg. Spamcop if you are using Spamcop\'s DNS server) and then the hostname in the hostname option. (eg. bl.spamcop.net which is the rDNS server for Spamcop).
Next, select Blacklist and Address-based. For Strictness level, Normal would be sufficient to prevent rejecting legitimate mails.
You may choose to Reject, Tag or Redirect the mail and then place the Rejection text (eg. Blocked by bl.spamcop.net) if you select Reject or Tag or an email address if you select Redirect.
Other more popular blacklist DNS server are Spamhaus (sbl.spamhaus.org) and DSBL (list.dsbl.org).
The mileage may varies and setting it too strict may cause legitimate mails to be undeliverable.
Another important portion of the Mercury SMTP Server section would be the Compliance tab. Here you can set policies to enforce restrictions on mail size, mails that have no subject, date, or non-MIME.
By selecting the first option (Require clients to use an ESMTP SIZE declaration) with ESMTP maximum size set to 5,000,000 bytes under the General tab, any mail exceeding 5 MB will not be accepted by the SMTP server. This will be useful in restricting large mails that may take up all your bandwidth and leaving little for other incoming mails.
However, there may be some older client software that may not support ESMTP size declaration and thus their mails may not get delivered even if it is below the specified size.
Another useful option would be (Limit maximum number of failed RCPTs to:) since setting this option low would restrict spammers that would try to generate tonnes of RCPT TO: command via phonebook method (eg. trying to send to [email protected], [email protected], [email protected], or to name like [email protected], [email protected], etc, etc) to locate legitimate email addresses on your server.