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What is 88.blocklist.zap and How to Get Unblocked

What is 88.blocklist.zap?

88.blocklist.zap is an internal ‘DNSBL (DNS blocklist)' maintained by Microsoft's Forefront Hosted Exchange service.  It resides on but is not publicly accessible. Note that it's actally called 88.blocklist.zap – not 88.blAcklist.zap.  

Yea, so how does this relate to my email being blocked by 88.blocklist.zap? The spam filter for Microsoft's Hosted Exchange service was formerly called Frontbridge (now called ‘Forefront’).  Their privately maintained blocklist is called 88.blocklist.zap.  Because the filter is private (and the query is encrypted) – neither you nor a blacklist monitoring service like www.mxtoolbox.tom can pro-actively monitor 88.blocklist.zap to see if you’re on the list.  The only way to find out if you're blocked is by checking your email server log files for a blocked message (see next section), or waiting until your users or recipients complain.

NOTE: If you're having issues with 88.blocklist.zap, you might also want to check our related article about 

If you go to – you’ll end up on Microsoft’s Hosted Exchange Server product page.

How To Tell if You're Being Blocked by 88.blacklist.zap

Since 88.blocklist.zap is a private blacklist maintained by Microsoft – you can’t query it to monitor delivery.  You’ll know you’re blocked in 1 of 3 ways:

  • A recipient actually reports to you that they are not receiving your email.
  • If your email is being blocked by recipients who are using Microsoft Hosted Exchange services, your IT team will see a log entry in your email server logs that looks like this:

failed,5.7.1 (delivery not authorized),smtp;550 5.7.1 : host xx.xx.xx.xx blocked using 88.blocklist.zap; Mail from IP banned. To request removal from this list please forward this message to delist.forefront -at-

  • When you send an email and are being blocked, you’ll receive an ‘NDR’ – Non-Delivery-Report’ email similar to the following:

Reporting-MTA: dns;
Final-Recipient: rfc822;

Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;550 Service unavailable; Client host [] blocked using 88.blocklist.zap; Mail From IP Banned To request removal from this list please forward this message to …

How to Get Removed from 88.blocklist.zap

As a reputable email sevice provider for businesses, we have to start with the obvious: be sure you're not spamming before you make your request.  If you or your company are sending unsolicited emails or general spam – you'll need to clean up your internal practices before taking the steps below, or Microsoft will either deny your request, or they'll process it and you'll find yourself being blocked again right away.  

That said, you can request removal you will need the following information:

  • Grab the log entry with the specific error message or the (NDR email if you received one) from your email server.  This might require assistance from your IT team.
  • You’ll need to know the IP address of your email server (the sending IP address).  You’ll probably need your IT team’s assistance to get this info.
  • You’ll also need the email address you sent the email to and the send-from.
  • Ideally,  include a copy of the email.

Once  you may request delisting by sending an email to delist.forefront -at- (of course replace the -at- with '@' to make it a real address) with the above information, including the IP address you want removed from their list. 

If you’re an actual Microsoft Hosted Exchange customer, you can also try sending the above info to: false_positive -at-

Another option to consider

If your marketing team is sending permission-based email campaigns and/or wants to automate your emails with autoresponders or trigger campaigns, consider using an email service provider like Pinpointe.  We deal with all this blacklist stuff 24×7 so you don't have to!  We maintain one of the highest inbox delivery rates for customers.  We monitor email delivery on an ongoing, real-time basis and ensure that you're opt-in emails won't get blocked.  You cna sign up for a free trial any time. Just go to:  Here's a valuable tip: you can get the first month of service 100% free if you enter code: PPTCOMP in our shopping cart.  Enjoy! 


Pinpointe's Blog Editor and contributor