If you’re receiving a bounce-back email from Hotmail/Outlook/Live with a message similar to the below:
Unfortunately, messages from [your IP address] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3150).
We can advise that - following numerous investigations - no Pentanet IPs appear on any known public blacklists or RBL lists. Instead, it appears that Microsoft and its related services have implemented aggressive anti-spam measures, and either the messages or the configuration of your email server match some unknown set of criteria that have resulted in blacklisting from Microsoft.
We suggest the following steps to resolve the blacklisting:
First, ensure that your on-premises email server is set up according to current best practices. Your IT professional should be able to assist, but at a minimum, we recommend implementing and checking the following:
- SPF and DKIM records that indicate your IP is allowed to send mail from your domain
- Ensuring your SMTP banner hostname contains the FQDN of your mail server
- Checking to make sure you’re not hosting an open mail relay
- A PTR record which resolves your sending IP to the FQDN of your MX record(reach out to the team at Pentanet and we will be happy to create a PTR record for you)
A simple way to check for the issues above (and more) is to use MX Toolbox’s free SMTP checking service: https://mxtoolbox.com/diagnostic.aspx
If you’re unable to check the items on the list above yourself, you’ll need to engage the help of an IT professional, as (with the exception of the PTR record, which we will be happy to create for you) any changes must be made in your on-prem mail server and your DNS hosting.
Once you’ve ensured that you’re following the best practices above, you can request that Microsoft review the block on your IP address by filling out the following form:
Allow at least one business day to receive a response from Microsoft. Typically the response will indicate that the IP address is ‘not qualified for mitigation’, but experience has shown that this seems to trigger some internal process that de-lists the IP, and email flow will resume.
Microsoft provides the following resources for email senders, and participation in these programs is recommended:
- Junk Email Reporting program (JMRP). When an Outlook.com user marks an email as “junk”, senders enrolled in this program get a copy of the mail forwarded to the email address of their choice. It allows senders to see which mails are being marked as junk and to identify mail traffic you did not intend to send. To join, please visit http://support.msn.com/eform.aspx?productKey=edfsjmrpp&page=support_home_options_form_byemail&ct=eformts.
- Smart Network Data Services program (SNDS). This program allows you to monitor the ‘health’ and reputation of your registered IPs by providing data about traffic such as mail volume and complaint rates seen originating from your IPs. To register, please visit http://postmaster.live.com/snds/.
Enrollment in the above appears to decrease the likelihood of being added to Microsoft’s blacklist, as they now have a mechanism through which to report issues directly to you.
Additionally, we recommend that any marketing emails are sent using services like Mailchimp or Sendgrid - both of which have generous free usage limits. Using these services means that you can outsource the complexity of email marketing and make sure that your outgoing marketing email is compliant with best-practices.
Other potential causes
If the issue keeps happening, or if you already follow the best-practices set out above but got blacklisted anyway, then one possibility is that there is a device or machine in your network that is sending spam email without you knowing. In this case, we’d recommend that you engage the help of an IT professional to block port 25 outbound in your network, and enlist a service like Spamhero (https://www.spamhero.com/plans) for your outbound emails.