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New problem solver


4 Messages

Saturday, February 5th, 2022 4:04 PM

Email to Hotmail gets rejected

I have a Comcast Business service with static IPs. My Comcast modem is set to a bridge-like configuration, without any NAT (I have my own router/firewall). I also have my own mail server.

Yesterday, I started getting email rejected by Hotmail. I did  not have problems in the past, and email sent to other locations is working (e.g., Gmail). This is part of the "Undelivered Mail Returned to Sender" message.[] said: 550 5.7.1Service unavailable, Client host [] blocked using Spamhaus.To request removal from this list see (AS3130). is NOT my IP address. It also NOT my Comcast Gateway (which is is a Comcast machine, apparently in a block of dynamic IP addresses, as the address is blacklisted by Spamhaus.

Using MxToolbox Blacklist check, I find that:

  • my IP addresses are not on ANY blacklists
  • is on two blacklists, including Spamhaus

Checking Spamhaus directly confirms the above.

I had a friend at a remote site (also on Comcast business) do a traceroute to the static IP address of my mail server. The last few lines of the trace are:

27 (  72.824 ms  72.788 ms  72.839 ms

28 (  80.586 ms  68.155 ms  68.081 ms

29 (  67.826 ms  67.870 ms  67.952 ms

30 (  86.232 ms  86.194 ms  86.164 ms

Traceroute never gets back to my IP address. It stops at - the address that Hotmail is complaining is blacklisted.

When I traceroute back to my friend's IP address (from my server), I don't see in ANY of the hops. The first few lines:

 1  _gateway (  0.222 ms  0.167 ms  0.257 ms
 2 (  2.356 ms  2.644 ms  4.297 ms
 3 (  13.461 ms  21.359 ms  20.202 ms
 4 (  23.654 ms  23.755 ms  24.012 ms
 5 (  23.256 ms  23.227 ms  23.170 ms
 6 (  19.961 ms  19.088 ms  19.396 ms
 7 (  22.471 ms  20.491 ms  21.209 ms
 8 (  21.884 ms (  15.875 ms  19.810 ms

In summary: traceroute from the outside to my static IP address stops at, but when I traceroute to an external IP address from my server, is not in the route.

Is Hotmail blocking my emails because it is doing something similar to a traceroute back to my email server, which stops at - an address that unfortunately is blacklisted by Spamhaus?

Why does traceroute to my static IP address from the outside stop at, and NOT my IP address?

How can this be fixed?

Accepted Solution

New problem solver


4 Messages

2 years ago

My IP address reverted back to my assigned IP block without any action on my part (checked with

I talked to Tier 2 tech support, and they said they had not made any changes.

So what went wrong and how was it fixed?

I was told that Comcast business modems with a static IP block connect to a specific gateway. If there is a problem connecting to this gateway, there is also a backup gateway that provides a dynamic IP address. It's likely that my modem failed to connect to the gateway providing static IP addresses, and switched to the backup gateway providing a dynamic IP address. Spamhaus, and other spam identifying sites, typically blacklist all dynamic IP addresses. So mail from my local server (from the dynamic address block) was getting rejected by sites that checked my IP address with Spamhaus (e.g., Hotmail).

Tier 2 support was guessing that my Comcast modem is at fault. Unfortunately the modem does not report or log failures, so no notification of these errors is sent to Comcast.

If this problem occurs again, I was told to reboot the modem (to see if it temporarily fixes the problem), and also report the problem to support, and they will replace the modem.

Official Employee


69 Messages

I truly wish I was able to determine what happened and how it was fixed but I'm happy to hear that the issue has been resolved for you! We truly appreciate your business and your time in reaching out to us. I want to make sure that you've been 100% taken care of. Did you have any other questions or concerns for us today? 

Recognized Contributor


13 Messages

2 years ago

Hi, @user_db24b3. Thank you for taking the time to reach out to us on our Business forums. Please send us a direct message with your full name, business name, full address, and phone number so that we can look into this further.


You can send us a direct message by clicking the "message" icon on the upper right page of our forum page. Once you click on that, input our shared handle (Comcast Business) to send us a private message.


• Click "Sign In" if necessary 
• Click the "Direct Message" icon in the top right corner
• Click the "New message" (pencil and paper) icon 
• The "To:" line prompts you to "Type the name of a person". Instead, type "Comcast Business" there 
• - As you are typing a drop-down list appears. Select "Comcast Business" from that list 
• - An "Comcast Business" graphic replaces the "To:" line 
• Type your message in the text area near the bottom of the window 
• Press Enter to send it

New problem solver


4 Messages

2 years ago

I filled out the information you requested. I was then instructed to call Comcast Customer Security (88) 565-4326. Talked to someone there for 10 minutes, who finally told me that I have the wrong number. He only deals with residential Xfinity service. I should call Comcast Business (800) 391-3000.

This is where I started out yesterday. First doing battle with a telephone robot just to get to an agent (no, please don't tell me about unrelated offers). After tens of requests to talk to an agent, I finally get through to someone whose English is hard to comprehend. After another ten minutes I'm told that I need Tier 2 support, where I am told that someone will get back to me in 48 hours.

I cannot send Tier 2 support any text details of the issue. So I came to this Forum, where I can post the error message I get from Hotmail. But now I'm instructed to go back to (800) 391-300.

New problem solver


4 Messages

2 years ago

One more data point, that points to a problem at Comcast.

When I check my IP address, e.g.,, I get:

This is NOT in my Comcast Business static IP address block (not even close)! It's also not my Gateway address to Comcast.

MxToolbox Blacklists test shows that is blacklisted by Spamhouse and SORBS. I'm guessing this is because is in a dynamic address block at Comcast, which are all blacklisted by Spamhaus.

When I send email to Hotmail, from a mail server in my Comcast Business static IP address block, I get the error:[] said: 550 5.7.1
Service unavailable, Client host [] blocked using Spamhaus.
To request removal from this list see (AS3130).

I'm guessing that Hotmail tries to identify where my email is coming from, and gets the IP address, which is blacklisted. So I cannot send email to Hotmail from my Comcast Business static IP addresses.

When I check my IP address from the outside, it should be MY IP address in the static IP address block I get from Comcast, and not a blacklisted IP address.

Looks like a Comcast error.