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Blocked outgoing email in Exchange RRS feed

  • Question

  • We are running SBS 2003 with Exchange Server 2003. The purpose of this question is more of an understanding how email works with exchange server. The problem started about two weeks ago when we switched ISP to Comcast. At that moment email was being blocked by Hotmail and Gmail. We would get the following message back.

    There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
    <nhnr.org #5.5.0 smtp;550 OU-002 (COL004-MC4F51) Unfortunately, messages from 73.189.0.xxx weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to <a href="http://mail.live.com/mail/troubleshooting.aspx#errors.>" target="_blank">http://mail.live.com/mail/troubleshooting.aspx#errors.>

    MXTOOLBOX says that the public ip address comes on one blacklist - Spamhaus ZEN.

    Here is the question. If I send mail through exchange to a hotmail account it will be bounced back and undeliverable with the above message. If I configure a POP account and send an email to hotmail it will successfully go through. Why does the POP work and not the Exchange?

    Thanks

    Chris

    Monday, April 20, 2015 7:42 AM

Answers

  • Actually, a static IP might fix part of the issue - I know some blocklists have included entire dynamic IP ranges, and you can't get them unblocked.  In my previous setup, I had to relay all mail through my ISP's SMTP server to bypass this.  I don't have to do that with my current static IP.
    • Marked as answer by agent086 Thursday, April 23, 2015 5:23 AM
    Monday, April 20, 2015 3:26 PM

All replies

  • Are you configuring the Exchange server as your SMTP server for your POP3 client?

    I'll also ask the following, to see if we can fix your Exchange delivery issue:

    Do you have an SPF record in DNS?

    Have you configured your system to send through the COMCAST SMTP systems, or are you sending direct using DNS? If so, is the IP address of this system included in the SPF record you have in DNS?

    If not, is your own IP address included in your SPF record in DNS?

    Have you contacts SpamHaus to determine why your IP address is on their blocklist?

    Monday, April 20, 2015 12:37 PM
  • Also, do you have a static IP from Comcast?

    That would probably not make any difference concerning the SpamHaus message but having an address allocated to you by your ISP's DHCP services is another cause of rejected messages (in addition to strict enforcement of SPF and simply being on a Blacklist).


    Please mark as helpful if you find my contribution useful or as an answer if it does answer your question. That will encourage me - and others - to take time out to help you.

    Monday, April 20, 2015 2:41 PM
  • Actually, a static IP might fix part of the issue - I know some blocklists have included entire dynamic IP ranges, and you can't get them unblocked.  In my previous setup, I had to relay all mail through my ISP's SMTP server to bypass this.  I don't have to do that with my current static IP.
    • Marked as answer by agent086 Thursday, April 23, 2015 5:23 AM
    Monday, April 20, 2015 3:26 PM
  • Thanks for the reply.

    Are you configuring the Exchange server as your SMTP server for your POP3 client?

    That is correct.

    I'll also ask the following, to see if we can fix your Exchange delivery issue:

    Do you have an SPF record in DNS?

    Yes we do have an SPF record setup in DNS zone file.

    Have you configured your system to send through the COMCAST SMTP systems, or are you sending direct using DNS? If so, is the IP address of this system included in the SPF record you have in DNS?

    The mail is setup to be sent through DNS.  Are you talking about the public IP address or the IP address of the server?  Either way neither of those IP address are in the SPF record. 

    If not, is your own IP address included in your SPF record in DNS?

    The mail is hosted by Bluehost and their IP address is in the SPF record.

    Have you contacts SpamHaus to determine why your IP address is on their blocklist?

    No I have not contacted SpamHaus yet.  I initially thought that is was Comcast problem but then they said it was not.  Anyway after trying different things I setup an email account (not using exchange) and was able to send email to the previously blocked addresses.  So I thought it might be some config problem.


    Tuesday, April 21, 2015 1:50 AM
  • No we have a dynamic address from Comcast.
    Tuesday, April 21, 2015 1:52 AM
  • OK, if you are sending using DNS, your SPF record needs to have your Exchange server's public IP address (or servers addresses) on it.  You can bypass this by saying that your MX record systems are also allowed to send. 

    If the Bluehost IP address is on the SPF record, then you may need to modify your SPF record so it includes your Exchange servers (per the previous sentence)

    As David and I said, an address from a static range may fix this issue. I found that one of the blocklist organizations (I don't remember if it was SpamHaus, but it may have been) had blocked entire ranges they knew to be dynamic, and there was no way to get individual IPs from those ranges unblocked. Due to this, I changed how I delivered so that all outbound email went through my ISP for delivery. I had to configure it so that the email was authenticated, but that wasn't hard - just use an account on the ISPs mail system and bounce the email off their SMTP server.

    Tuesday, April 21, 2015 1:34 PM
  • And if that does not work (one ISP we used stopped allowing this), you could use a service from No-IP that allows you to send through their network (for a fee). There are probably other companies that offer this type of service as well. I'm naming No-IP simply because it is the service I use. It is probably better known for its dynamic DNS service which works for sending mail in the other direction (apparently not a problem for you?).

    Please mark as helpful if you find my contribution useful or as an answer if it does answer your question. That will encourage me - and others - to take time out to help you.

    Tuesday, April 21, 2015 2:06 PM
  • Ok thanks I appreciate the feedback.  I think the problem is the dynamic IP address.
    Tuesday, April 21, 2015 11:46 PM
  • Many of the anti-spam providers I have worked with in the past will simply not accept mail from a dynamic public IP address. It is always a good idea to upgrade to using a static IP address if you are going to send mail from an internal server, or use a 3rd party smart host to relay mail through. All though even then, you typically need to specify what IP addresses can use that smart host, and with a dynamic IP it could change at any time and interrupt mail flow.
    Wednesday, April 22, 2015 12:51 AM
  • I'd suggest you ask Comcast if you can forward your outbound email through them.  This will allow you to retain the dynamic address.  If they allow it (and most, if not all, ISPs will allow this), make sure you add their IP address to your SPF record in DNS.
    • Marked as answer by agent086 Thursday, April 23, 2015 5:22 AM
    • Unmarked as answer by agent086 Thursday, April 23, 2015 5:23 AM
    Wednesday, April 22, 2015 11:55 AM
  • That is eventually what I did to resolve the problem.  I used a smart host to forward email through.  That public IP address was the problem.
    Thursday, April 23, 2015 5:15 AM
  • I did talk to Comcast a couple of times.  Really depends upon who you talk to their sometimes they are useless or I suppose misleading.  First time they said I had a virus that was sending out SPAM.  I didn't think that was the case.  Called back a few days later and a different person said it was most likely the IP address.  I could either get a static or they offer some other webmail product.  According to them they do not offer any type of a smart host.  Anyway I ended up using Bluehost as the smart host and changed the port and no more blocked email. I appreciate the feedback from everybody because it put me on the right track to be able to solve this problem.
    Thursday, April 23, 2015 5:22 AM