Hi @System Testing ,
Thanks for reaching out.
Generally, these issues occur while using trail tenant to perform this send email activity using Graph API. Could you please share the complete NDR information you received, note to clear your personal information.
For example, if you receive the code like Remote Server returned ‘550 5.7.520 Access denied, Your organization does not allow external forwarding.
Then you will need to enable the forwarding.
This error occurs when sending email from known, low reputation IP addresses that are typically used by new customers on Trail tenants.
To fix this issue your Tenant needs to be added to a sending exclusion for 60 days to allow them to use blocked IPs whilst they still have a new/trial license.
If that's not your case, please refer to this: How to set up a multifunction device or application to send email using Microsoft 365 or Office 365
You could check which option you are using now, make sure you have meet all needs to use this option.
Please raise a case with MS Support team from your tenant where you are facing issue, respective team will provide exclusion for your tenant.
Hope this helps.
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