I am not aware of the reality why the emails are not being sent, and it keeps on sending back the same to my Microsoft Outlook account with a non-delivery report. Whatever is the root cause, it was drastically irritating. Even I tried to find the real cause to it but I failed to an extreme level. I left trying and left the issue for an auto-fix. But it became difficult to face the same error every time. As an enormous number of the mail non-delivery report kept on flooding my mailbox.