Why?

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Delivery has failed to these recipients or groups:

A Better Routeplanner (a-better-routeplanner@upvoty.com)
Your message couldn't be delivered. When Office 365 tried to send the message, the external email server returned the error below. This is probably due to a problem or policy setting on the recipient's email system.

Check the error for information about the problem. The error might tell you what went wrong and how to fix it. For example, if it says the message was blocked due to a potential virus, try sending the message again without attachments.

For Email Admins
This error was reported by an email server outside Office 365, and if the sender is unable to fix the problem by modifying their message then it's likely that it's a problem that only the recipient's email admin can fix.

Check the error for information about the problem. The error might tell you what went wrong and provide clues for how to fix it. For example, if it says the message was rejected due a Sender Policy Framework (SPF) issue then you'll have to work with your domain registrar to properly configure your SPF record.

Check the error for information about where the problem is happening. For example, look for a domain name like contoso.com. This tells you which organization was responsible for the error. The recipient's email server could be causing the problem, or it could be due to a third-party service that your organization or the recipient's organization is using to process or filter email messages.

If you can't fix the problem, contact the responsible party's email admin. This could be the recipient's email admin, your smart host service admin, or someone similar. Provide them with the error code and error message from this non-delivery report (NDR) to help them troubleshoot the issue.
Unfortunately, Office 365 support is unlikely to be able to help with these kinds of externally reported errors.