When trying to send email to clients who use Hotmail/Outlook/etc type email addresses, I get the following error:
SMTP error from remote mail server after RCPT TO:some@email.com: 550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [HE1EUR01FT049.eop-EUR01.prod.protection.outlook.com]
or
SMTP error from remote mail server after pipelined MAIL FROM:some@email.com SIZE=2699: 550 5.7.1 Unfortunately, messages from [IP ADDRESS] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).
My email is hosted on a cPanel shared server, most of the domains are under my control. When checking the server IP address, I can find no blacklisting.
I have contacted Microsoft who comes back with an unhelpful canned message basically stating that they can not help.
So what it seems is that an IP in the same network as ours, is blacklisted, and for this reason, our mail is being blocked.
(DKIM/SPF are all installed).
Can anyone advise how to diagnose and find the culprit causing our server IP address to be blocked?