Unable to send email using Graph API from instant sandbox created with Office 365 Dev Center

Question

Hello Team,

We have registered app on Microsoft Entra Admin Center with "Mail.Send" Permission to users.
For that we also created a domain using instant sandbox settings.

But still, we are unable to send email through C# code, getting below message in email delivery.
"Your message wasn't delivered because the recipient's email provider rejected it"

Note: We are trying to send single email at time through code.

Answer 1

Hello hgcortex,

I understand you’re trying to send an email using Microsoft Graph API from an Office 365 instant sandbox and received a message indicating that the recipient’s provider rejected it.

This is common in tenants created through the Microsoft 365 Developer Program. Sandbox environments use shared Microsoft IP addresses, which are often not trusted by external mail systems. A typical result is error 550 5.7.708, which points to low IP reputation.

In some cases, outbound messages are also blocked by default anti-spam policies that set message limits to zero.

You can review and adjust these settings in the Microsoft 365 Defender portal by editing the outbound policy under anti-spam settings.

Raising the limits to more practical values may help, though it can take up to 24 hours for the changes to take effect.

If the issue continues, it’s likely due to limitations of the sandbox itself. For consistent external email delivery, it’s recommended to use a licensed Microsoft 365 tenant with a verified domain and properly configured DNS records such as SPF and DKIM.

For more information, refer to Microsoft Q&A

Hope this helps.

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