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.
Why you should use DNS blacklist monitor software
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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