This isnt a Graph Issue. You are being blocked because Exchange Online doesnt allow email from IPs it considers untrustworthy
https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/fix-error-code-5-7-700-through-5-7-750
5.7.708 Access denied, traffic not accepted from this IP: This error occurs when sending email from known, low reputation IP addresses that are typically used by new customers.
5.7.708 Access denied, traffic not accepted from this IP
This error can happen when you are trying out a Microsoft 365 trial tenant. If you receive this error before you can purchase licenses, contact support to request an exception for the low reputation IP address until you're able to purchase licenses.