In this post, let’s have a look at the error message “5.7.506 – Access Denied, Bad HELO” that you get when working with Microsoft 365 Exchange Online.
These are the errors that you usually receive when there is a problem in delivering the email message that was sent by you. These errors are generated and sent by Office 365. This is usually alled as DSN which stands for delivery status notification or in simple words bounce messages.
NDR or Non-delivery report is one of the common type of message that you receive which indicates to the user that the email was not delivered.
5.7.506 – Access Denied, Bad HELO
Your server is attempting to introduce itself (HELO according to RFC 821) as the server it’s trying to connect to, rather than its own fully qualified domain name.
This isn’t allowed, and it’s characteristic of typical spambot behavior.