Every so often you'll receive the an "Undeliverable" message from Postmaster@ telling you that your message has not been delivered.

These messages can often seem cryptic and uninformative, however it's just a case of knowing what to look for.

There are a few different styles of NDR, however the general format is usually similar to:

[Line telling you that the message has failed to be delivered]

[Recipients who did not receive your message]

[Plain text reason saying why the message was not delivered]

["Diagnostic information for administrators:"]

[Recipients who did not receive your message]

[A more technical reason for the message delivery failure - Often including a code]

[Message headers]

Office 365 NDR emails tend to look slightly different, but the basic information is still there. 

Here are some more examples of NDR emails:

In this case it looks like the recipient "anna@contoso.com" has updated their email address, and your outlook client is still using the old routing information; follow the steps in the "How to Fix It" section to resolve this issue.

In this example the email address does not exist on the recipients mail servers.

In this example, Our mail server "EXCA3.vo.spidergroup.co.uk" has been repeatedly trying to deliver the email, however it has eventually timed out. As you can see, the reason it's giving is that there is no mailbox database (the database that holds all the user mailboxes) containing a mailbox with this address.

In this example, the recipient was a group, which did not accept external emails - Only members of the group were allowed to send it emails (this is often used for "all employees" groups etc. where you wouldn't want all staff to be emailed by an external company) So the email was denied.

In many of these cases, you can get more information by searching for their error code + error wording + "Bounceback" or "NDR" for example:

"550 5.7.1 not authorised bounceback" returns a microsoft technet article as the top result, with troubleshooting steps including "Do you have permission to send to the group"

If you ever receive any errors that don't make sense or you are not able to diagnose after searching, feel free to give out SpiderTech support team a call on 01179330570.