Blocked by Microsoft

I recently set up my Mail in a Box and everything has been working swimmingly for the past couple days. However this evening I attempted to send an email to an Exchange server and the email was rejected with a message like this,

    Action: failed
Status: 5.7.1
Remote-MTA: dns;
Diagnostic-Code: smtp; 550 5.7.1 Service unavailable; Client host
   [] blocked using FBLW15; To request removal from this list
   please forward this message to

Has anyone encountered something like this before? Is there anything that can be done on our end to prevent this from happening?


Hi, i was curious so i looked at this problem and you are not alone.

  1. Check if you have right PTR record.
  2. Check if your domain is not listed on blacklists.
  3. Looks like Microsoft blocking the email. Solution can be contact them?

Thanks @radek for looking into this. My system status looks totally fine AFAIK. I don’t have a certificate for the box subdomain, but I don’t believe that is the issue. My guess would be that I was blocked because I sent several test emails from my Mail in a Box account to my account on the same Exchange server – I suppose I should have made them more realistic.

I’ve sent an email to MS requesting that I be delisted from their ban-list. That request “will be responded to within 24 hours” so I’ll be sure to update this thread.

I received a response and was delisted. I was able to send an email to my account on the Exchange server without having it blocked outright. However, it was flagged as Junk. I reported it as Not Junk and I can add my Mail in a Box account to my safe senders list. But, it seems like it is likely that any mail I send from this account will be either blocked or flagged by MS.

Is verifying the PTR record part of the System Status Check for Mail in a Box?

Regarding still being blocked as spam - don’t worry. Send some email to people on those services and it’ll work itself out. At first, my server’s email was going to spam, but now that I’ve sent some email to Microsoft users, Gmail users, etc., it tends to work without my mail getting blocked as spam now.

I am also having this issue. Nobody else (Google, Apple, FastMail, etc) have had any issues with my MIAB server. Even an email to MS’s delist address is bounced.

<>: host[] said:
   550 5.7.511 Access denied, banned sender[x.x.x.x]. To request
   removal from this list please forward this message to For more information please go to AS(508)
   [] (in reply to RCPT TO

I have forwarded it again using another email address. Hopefully this will get resolved soon.

Ah is microsoft using now a delist option …

A few weeks back I added my IP with mailinabox to the SNDS option of Microsoft
here you can track spam notifications towards your ip(s)

Than if you want to be unblocked you had to fill in this ‘rather pretty difficult to find’ website

In the end it resolved it for me, but if they use now a delist option, It would be pretty much better.