Microsoft 365 emails blocked by Plusnet dynamic IPs being on Spam blacklist

[#1]

I am a new employee of a startup biotech company that uses Microsoft 365 corporate account to run their email servers.  When logged in at home I am unable to send outgoing emails from the company account due to this error.

Remote Server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653 AS(7230) [CWLP123MB3025.GBRP123.PROD.OUTLOOK.COM]'

Looking at my dynamic IP it appears on MXTools to flag on the Spamhaus blacklist, and when checked there it appears this is part of a Policy Blocklist by Plusnet.  The policy is below.

Outbound Email policy of Plusnet Plc for this IP range

It is the policy of Plusnet Plc that unauthenticated email sent from this IP address should be sent out only via the designated outbound mail server allocated to Plusnet Plc customers. To find the hostname of the correct mail server to use, customers should consult the original signup documentation or contact Plusnet Plc Technical Support.

So it seems to me that Plusnet is deliberately making it difficult to utilise non-Plusnet mail servers on their dynamic IPs.  When I contacted support they said they couldn't help and to go to the community forums so here I am hoping for answers.

As a possible solution I did request a static IP for my account if possible, and in fairness they agreed to waive the one time setup fee.  However they said they tried to apply a static IP to my account three times and it failed each time.  And that was it. So if anybody could help with this issue would be very grateful.  Surely I cant be the first person to run into this problem if Plusnet themselves have listed their dynamic IPs on a blacklist.

[#2]

Moderator's note(s):

Thread moved from Broadband to Email.

[#3]

@GMC being on the PBL should not prevent you sending mail via the company mail server. If it does,then the mail server is incorrectly using the pbl. The pbl is ONLY  to prevent you sending mail DIRECTLY  using your own email server. See here https://www.spamhaus.org/pbl/ particularly the caution note at the end

Edit: in any case, I'm surprised that any company would allow sending via their server without use of a vpn

[#4]

Also, would the OP have to authenticate to be able to send company emails via (presumably) the Plusnet connection, or have I got my thoughts mixed up?

[#5]

@MisterW is correct. It is entirely intentional that our dymanic IP ranges feature on the PBL list. This should only ever cause a problem if you try sending email via direct SMTP connection from your IP i.e. not via another outbound mail server.

If you are tyring to send via another outbound mail server and it is grumbling about the IP we're assigning you, then the mail server is incorrectly configured.

[#6]

Thanks everyone for the quick replies.  I have flagged this thread to our IT support company and hopefully your information can help to get to the bottom of this problem.

Any thoughts on why my account couldn't be set up with a static IP?

[#7 · Accepted solution]

@GMC - I've assigned you a static IP. You'll need to disconnect/reconnect for it to be applied. For verification purposes, it's in the 84.92.0.0/16 range.

[#8]

Thank you Bob for setting up the static IP.  I can confirm that having a static IP has solved the problem although I appreciate that there was something strange going on with the dynamic IP addresses flagging.

Thanks all for your helpful feedback and suggestions.  Hopefully this thread can help others if they run into this problem.

[#9]

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