Before the Christmas holiday, some of our clients using Linode cloud servers reported that emails cannot be delivered to some domains hosted at Microsoft Office 365 from their Postfix SMTP server.
We checked that all of them are still in good reputation and not listed in other email block lists, but only Microsoft Office 365 systems returns a message “550 5.7.511 Access denied, banned sender”.
550 5.7.511 Access denied, banned sender[x.y.z.205]. To request removal from this list please forward this message to [email protected]. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. AS(XXXX) [XXXXXXXXXXXX068.eop-NAM04.prod.protection.outlook.com] (in reply to RCPT TO command))
We confirms that email servers of our clients are running well and not sending spam. We tried the following auto methods listed on Microsoft pages but it couldn’t be solved. And no human contact or email is shown that we can contact with.
- Use the Microsoft delist portal to remove from the blocked senders list.
The issue is posted on Microsoft Q&A Forum which requires all posters to create an Office 365 account. And we found that Microsoft 365 also blocked other good reputation email systems worldwide in every Linode datacenters before Xmas and is still not resolved.
We decided to set up additional Postfix SMTP servers at Digital Ocean as a relay server for outgoing emails for our clients. It is unacceptable that a problem raised by Microsoft causes us and our clients additional expenses for a half month. It is also tweeted today, and we hope that this issue will be solved asap.