Send Email Issue

Question

I am trying to send mail through UI and it is working fine but when I use API I get 202 response but get the message as 'Your message wasn't delivered because the recipient's email provider rejected it'. For same receiver email UI is not giving error but API is giving error. I have raise ticket to Microsoft Graph team regarding this, they said me to raise ticket to Microsoft 365 to get the message trace or extended message trace and check why my emails are rejecting. BTW the account I am using is with something.onmicrosoft.com domain.