Hi @srikanth a ,
You need to check whether you have correctly configured SPF/DKIM/DMARC. refer the following URLs.
https://learn.microsoft.com/en-us/defender-office-365/email-authentication-about
Configure SPF - https://learn.microsoft.com/en-us/defender-office-365/email-authentication-spf-configure
Configure DKIM - https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dkim-configure
Configure DMARC - https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dmarc-configure
Ensure you have all these configurations are applied.
Hope this helps