DNS blacklist (DNSBL) monitor software is used to monitor your email server public IP address in DNS blacklist zones like zen.spamhaus.org and many others.
Blacklist monitor software lets e-mail system administrators know before users told them that there is a problem in outbound e-mail messages because DNS blacklist monitor software continuously checks if your public IP address is listed in any DNS blacklist and if public IP address of email server has been accidentally registered in any DNS blacklist zone, DNS blacklist monitor software will send a notification to the administrator to take action about this issue and minimize down time for outbound email messages.
Of course some outbound messages may be rejected by recipients which depends on what DNS blacklists used by recipients Anti-Spam and time taken to resolve the issue.
The below flow chart show how your outbound email messages will be checked for spam on the recipient email server side (Image from spamhaus).
Why getting blacklisted in DNS blacklist zone is dangerous
Because if you are not using early notification system like DNS blacklist monitor software you may not know the issue unless some users begin to complain and in this case you will lose a lot of email outbound messages which are not recoverable and users have to send these emails again. for example if your email server public IP address accidentally blacklisted at 9 AM and you know about the problem at 11 AM then begin to troubleshoot and resolve the issue at 11:30 AM.
so you loose outbound messages between 9 AM and 11:30 AM and you have to told users about this issue to resend their emails if it didn't reach recipients.
you will also note that some recipients may send you a rejected message and others may not.
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